There are literally hundreds of afflictions that can affect the health of your fish. The most common maladies seen in home aquaria are usually either bacterial or parasitic in origin. Fungal infections are also sometimes seen, and occasionally physical ailments.

   Luckily, most fish ailments are easily diagnosed and can be treated with success. The most common of these afflictions are included here. How to prevent fish disease has steps you can take to reduce the possibility of disease and help to keep disease from spreading if it should occur. A table of contents is provided along with a diagnostic chart with links to appropriate medications.

   Understanding how an aquarium and its filtration work to support aquatic life is vital in preventing fish ailments. The basics of life support are the same whether you have a freshwater aquarium, saltwater aquarium, or a mini reef.

Table of Contents

   Introduction to diseases and their treatment.
   Click on each name in the table to learn:

  • What causes that disease?
  • What are the symptoms?
  • How to treat it?

Types of Fish Disease

   Fish ailments can be separated into 4 general types including bacterial infections, fungal infections, parasitic or protozoan infections, and physical ailments and wounds.

  • Bacterial Diseases: Bacterial diseases are usually characterized by red streaks or spots and/or swelling of the abdomen or eye. These are best treated by antibiotics such as penicillin, amoxicillin, or erythromycin.
  • Fungal Diseases: Comon fungal infections often look like gray or white fluffy patches. are
  • Parasitic Diseases: The most common parasitic disease called “Ich” can be treated most effectively with copper or malachite green in the right dosage. Most treatments will have copper as an ingredient. Many water treatments like “Aquari-Sol” will also contain copper as an ingredient. If the treatment you use is an anti-biotic or copper based, remember to remove all carbon from the filtration system.
  • Physical Ailments: Physical Ailments are often the result of the environment. Poor quality water conditions can lead to fish gasping, not eating, jumping out of the tank, and more. Tank mate problems can result in nipped fins and bite wounds.
Goldfish suffering from dropsy disease
Image Credit: Pavaphon Supanantananont, Shutterstock

How to Prevent Fish Diseases  

Some steps can be taken to reduce the possibility of your fish getting a disease. Following these precautions can also help keep fish diseases from spreading if they do occur.

  • Buy only good-quality, compatible fish.
  • Quarantine new fish before adding them to the aquarium. (A hospital tank can be used for this).
  • Avoid stressing the fish with rough handling, sudden changes in conditions, or “bully” tankmates.
  • Don’t overfeed your fish.
  • Remove sick fish to a hospital tank for treatment.
  • Disinfect nets used to move sick fish.
  • Don’t transfer water from the quarantine tank to the main aquarium.
  • Don’t let any metal come in contact with the aquarium water.
  • Anti-biotics: When using any anti-biotic, make sure the biological filtration in your aquarium is not destroyed. You want to be certain the treatment does not kill the nitrifying bacteria in your system at the same time it attacks harmful bacteria on your fish. Although most of the treatments available at the store state that they will not harm your biological filter, sometimes they wil. It is best to either monitor your ammonia and nitrite levels, or use an ammonia remover such as “AmQuel” to be sure your levels of ammonia don’t become a problem.
  • Copper Treatments: When using any medication which has copper as an ingredient, be aware that most plants will not do as well. Invertebrates, such as snails, can also be killed if the amount of copper is sufficient.  Indeed, most snail removers are copper based.

Diagnostic Chart

   If you notice something is wrong with your fish, a proper diagnoses is usually all you need to worry about. In most cases, a proprietary treatment (fish medications) purchased at a pet store will work very well.

   Steps to diagnosing and treating your fish:

  1. Symptom: Match the symptoms your fish is showing with those listed under the ‘Symptom’ column.
  2. Possible Cause: Learn about each disease by clicking on the link under ‘Possible Cause’.
  3. Medication: Find the treatment for the disease under the ‘Medication’ column.
  4. Product Link: Use the ‘Compare Prices’ column to compare similar products and merchant’s prices.
SymptomPossible CauseMedication 
Small white spots on fins / skin, clamped finsIchCoppersafe, Quick-Cure
Ich-Ease, Aquari-sol, Cure-Ick, Super Ick Cure
Ick medication
Ick Medications
Peppery coating, yellowish, clamped finsVelvetCoppersafe, Quick-Cure
Aquari-sol, Cure-Ick, Super Ick Cure
Velvet medication
Gray or white fluffy patchesFungusMaracyn, Fungus Cure, Methyl Blue, Antibiotics for secondary infections.Fungus medication
Fungus Cure
Gray or white fluffy patches around mouthColumnaris, Mouth Fungus

Erythromycin, Kanacyn, Fish Pen (penicillin), Maracyn
Antibiotics for secondary infections. (Use Maracyn simultaneously with Maracyn II)

Fish Antibiotics
Fish Antibiotics
Pale appearanceNeon Tetra diseaseNo Known Cure 
Unusual racing around tank.
Black to red nodules beneath skin.
FlukesParagon, Clout, Proxipro, Fluke-TabsFlukes medication
Fish Flukes
Milky cloudiness on skinCostia, ChilodonellaCoppersafe,  Quick-Cure, AcriflavineFish Antibiotics
Fish Antibiotics
Destruction of fins or tailTail or fin rotMaracyn, Methylblue, Organi-Cure, Antibiotics, Tetracycline, ChloromycetinTail Rot and Fin Rot Medication
Tail Rot / Fin Rot Medication
Red streaks on bodyRed pest, Fin rot

Tetracycline, Penicillin. Acriflavine, Chloromycetin, Fish Pen (penicillin)

Fish Antibiotics
Fish Antibiotics
Yellow to black nodules on skinIchthyosporidium
Ulcerated patches on skinRed pest, Ichthyosporidium
Emaciation, hollow belly, possibly soresTuberculosisNo Known Cure 
Protrusion of scales with bloated bodyDropsyFeed Anti-Bacteria medicated foodAnti-Bacteria medicated food
Anti-Bacteria Medicated Foods
Protrusion of scales, body normalScale protrusion
Eyes protrudePop eyePenicillin or amoxicillinFish Antibiotics
Fish Antibiotics
Cloudiness of eyesEye problems,
Ich, Velvet
Maracyn, Maracyn Plus, Antibiotics for bacterial infection, Increase vitamin A.Bacterial Antibiotics
Bacterial Antibiotics
Hole in head, ulceration of lateral line, loss of appetiteHead and Lateral Line Disease
Paragon, Hole N Head Guard, Hakari Hole in the HeadHole in the Head Medications
Hole in the Head Medications
 White slimy feces, loss of appetite, swim backwardsHexamitaMetronidazoleHexamita Medications
Hexamita Medications
Crustaceans on skinArgulus, ErgasilusParagon, Trifon, Anti-Fluke treatmentAnti-Fluke treatments
Anti-Fluke treatments
Flukes on skin or gillsFlukes
Worms hanging from anusNematodaParagon, Trifon, Worm Parasitic treatmentWorm Parasitic treatment
Worm Parasitic treatment
Heart shaped wormsLeeches
Nodular white swellings on fins or bodyLymphocystis, Glugea, HenneguyaNo Known Cure. Since lymphocystis is not harmful and will drop off after some time, no cure is necessary. 
Glancing off rocks or plantsVelvet, Ich, Flukes, Anchor worm, Chilodonella, CostiaIch Medication (Ich)
Paragon (Worms)
Ick and worm medication
Ick and Worm Medications
Severe loss of balanceSwim bladder diseaseCheck aquarium parameters, look for signs of other disease. 
Gasping at surfaceOxygen/ O2 deficiency, CO2 excess, tank too hot, toxins, shockOxygenex, Oxygen stones – (short-term).
Provide better water circulation, lower temperature
Oxygen Aquarium Aids
Oxygen Aquarium Aids
Jumping out of waterpH wrong, toxinsCheck for ph extremes, do water changes with dechlorinated water.Ph Buffers
Ph Buffers
Automatically set & stabilize your aquarium pH . Remove chlorine & detoxify heavy metals.
Appetite dwindles, belly swells, feces trailsConstipation or Internal ParasitesMedicinal parrafin oil, change in diet,
Anti-Parasitic Medicated Food
Anti-Parasitic medicated food
Anti-Parasitic Medicated Foods
Fins frayed or split, scales missingInjuries

Wound Treat, Bio Bandage, Stress relievers.
Look for and remove bully fish.
Medications: Melafix

Wound Treatments
Wound Treatments

Bacterial Diseases

Red Pest

  • Symptoms: Bloody streaks on fins or body.

Red Pest is called such because of bloody streaks that appear on the body, fins and/or tail. These streaks could proceed to ulcerations and possibly lead to fin and tail rot with, in severe cases, the tail and/or fins falling off. As the disease is internal, external treatments are usually not effective, except in very slight cases. In slight cases, treat the aquarium with a disinfectant and clean the aquarium as best as possible. Do not feed a lot while the aquarium is being treated. To disinfect, use acriflavine (trypaflavine) or monacrin (monoaminoacridine) using a 0.2% solution at the rate of 1 ml per liter. Both disinfectants will color the water, but the color disappears as the disinfectants dissipates. If the fish do not appear to respond favorably, discontinue disinfections.
     Then add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. Antibiotics usually come in 250 mg capsules. If added to 25 grams of flake food, one capsule should be enough to treat dozens of fish. A good antibiotic is chloromycetin (chloramphenicol). Or use tetracycline. If you feed your fish frozen foods or chopped foods, try to use the same ratio with mixing. As a last resort add at most 10 mg per liter of water.

Columnaris – Mouth Fungus

  • Symptoms: cottony patches around the mouth, White spots on mouth, around the chin and mouth area, edges of scales and fins, cottony patches around the mouth. May be accompanied by clear stringy feces, a loss of appetite, and rapid gilling where gills are infected.

  Names Columnaris is known by are Mouth Fungus, Cotton-Wool, Cotton-Mouth, Mouth-Rot, Saddle Back,  Flexibacter, False Neon Disease, and Guppy Disease.
  It is often called Mouth Fungus because it looks like a fungus attack of the mouth. It is actually caused from the bacterium Flavobacterium columnare, previously called Flexibacter columnaris, Bacillus columnaris, Chondrococcus columnaris, and Cytophaga columnaris. This is a common bacterial infection that affects freshwater aquarium fish, particularly livebearing fish and catfish. It is not seen in marine fish, they can be infected by myxobacterial diseases that are similar to columnaris, yet this is very uncommon in the aquarium.
  Columnaris can enter the fish through the gills, mouth, or small wounds on the skin and results in an internal or external infection. It can have either a chronic progression of days or months or an acute progression with lesions spreading quickly, often wiping out whole populations of fish in just a few hours. It is highly contagious and may be spread through contaminated nets, specimen containers, and even food.
  This disease is brought on by stress, injury, inadequate diet, and poor water quality, including an unstable pH. To prevent Columnaris maintain your water with good biological filtration and weekly water changes that include vacuuming the substrate. Keep the tank well aerated, provide your fish with a varied diet, and don’t overstock.

  Columnaris generally shows up first as a gray or white line around the lips and later as short tufts sprouting from the mouth like fungus. This bacterium produces protein and cartilage degrading enzymes that eat away at the fish and forms round or oval shapes with an open ulcer in the center. It may affect the fins, beginning with degradation at the edges, or as a lesion near the dorsal fin. The “saddleback” condition is a discolored gray patchy area near the dorsal fin and a pale white band encircling the body of the fish. A yellowish-brown ulcer develops in the center as it progresses. This coloring is caused by detritus particles trapped in the slime produced by the 
  This is a quick acting disease and needs immediate treatment. The toxins produced and the inability to eat will be fatal unless treated at an early stage. This bacteria is often accompanied by a second infection of an Aeromonas bacteria and fungus often invades the affected skin. Be aware that some strains of this bacteria are becoming resistant to antibiotics. So ensure you treat for the full length of the medication. To rid the aquarium and fish of this disease, first increase the water quality and then begin treatment.

  You can treat Columnaris with a gram-negative medication. However, other bacteria that are gram positive mimic the Columnaris Disease, so if you use a gram positive treatment and it worked, the affliction was NOT Columnaris disease. Some aquarists suggest using both the gram positive and negative together just in case you are not sure.

     Several types of antibiotics and medications can used to treat Columnaris:

  • Penicillin: Penicillin at 10,000 units per liter is a very effective treatment. Treat with a second dose in two days.
  • Chloromycetin: You can use chloromycetin, 10 to 20 mg per liter, with a second dose in two days.
  • Kanacyn (kanamycin): Kanacyn will treat both bacteria at once.
  • Maracyn (erythromycin): Maracyn is effective against Columnaris, and using Maracyn 2 (minocycline) in conjunction with it will treat the Aeromonas bacteria as well.
  • Others: Copper sulphate, Furan, Tetracycline, and Potassium permanganate. Nifurpirinol, Acriflavine, Chloramphenicol and Malachite green are also said to be effective.
  • Medicated Foods: Feeding food dosed with Terramycin (Oxytetracycline) will help to internally treat this disease.

Tuberculos – Mycobacteriosis Syn: fish tuberculosis, piscine tuberculosis, acid-fast disease, granuloma disease.

  • Symptoms: Emaciation, hollow belly, possibly sores.

   Tuberculosis is caused by the bacterium Mycobacterium piscium. Fish infected with tuberculosis may become lethargic, hollow bellied, pale, show skin ulcers and frayed fins, have fin and scale loss, and loss of appetite. Yellowish or darker nodules may appear on the eyes or body and may deform the fish.
     The main causes for this disease appears to be over crowding in unkempt conditions; ie. poor water quality. All fish species could be susceptible though some are more susceptible than others. Those most susceptible are the labyrinth air breathers like the Gouramis, Bettas, and Paradise Fish. Others include Neon Tetras, Discus, and the Ram Cichlid.

  • There is no absolute treatment. However the most effective treatment known for this disease is to treat with Kanamycin and Vitamin B-6 for 30 days. Kanamycin can be purchased at your local fish store. Liquid baby vitamins work well as s Vitamin B-6 source. They are available at your local pharmacy. Add one drop per every 5 gallons of aquarium water during treatment.
  • If the treatment is ineffective, the best thing to do is destroy the infected fish.
  • If either unkempt conditions or over crowding are the suspected cause, correct the condition.

      It is possible for humans to contract this disease so we recommend using caution when dealing with it. Humans are very rarely are at risk from aquariums though. It is more common to contract this disease from public swimming areas or as a food contaminant.


  • Symptoms: Bloating of the body, protruding scales.

     Dropsy is caused from a bacterial infection of the kidneys, causing fluid accumulation or renal failure. The fluids in the body build up and cause the fish to bloat up and the scales to protrude. It appears to only cause trouble in weakened fish and possibly from unkempt aquarium conditions.
     An effective treatment is to add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. Antibiotics usually come in 250 mg capsules. If added to 25 grams of flake food, one capsule should be enough to treat dozens of fish. A good antibiotic is chloromycetin (chloramphenicol). Or use tetracycline. If you feed your fish frozen foods or chopped foods, try to use the same ratio with mixing. As a last resort add at most 10 mg per liter of water. Also, if unkempt conditions are the suspected cause, correct it.

Scale Protrusion

Symptoms: Protruding scales without body bloat.

     Scale protrusion is essentially a bacterial infection of the scales and/or body. A variety of bacterium could be the culprit here, as can unkempt aquarium conditions.
     An effective treatment is to add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. Antibiotics usually come in 250 mg capsules. If added to 25 grams of flake food, one capsule should be enough to treat dozens of fish. A good antibiotic is chloromycetin (chloramphenicol). Or use tetracycline. If you feed your fish frozen foods or chopped foods, try to use the same ratio with mixing. As a last resort add at most 10 mg per liter of water. Also, if unkempt conditions are the suspected cause, correct it.

Symptoms: Disintegrating fins that may be reduced to stumps, exposed fin rays, blood on edges of fins, reddened areas at base of fins, skin ulcers with gray or red margins, cloudy eyes.

     Tail and fin rot appears to be a bacterial infection of the tail and/or fins and may be caused by generally poor conditions, bully, or fin nipping tankmates. If aquarium conditions are not good an infection can be caused from a simple injury to the fins/tail. Tuberculosis can lead to tail and fin rot. Basically, the tail and/or fins become frayed or lose color. Over time the affected area slowly breaks down.
     First, attempt to ascertain the cause. Then treat accordingly. Also, treat the water or fish with antibiotics. If added to the water, use 20 – 30 mg per liter. If the fish is to be treated add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. Antibiotics usually come in 250 mg capsules. If added to 25 grams of flake food, one capsule should be enough to treat dozens of fish. A good antibiotic is chloromycetin (chloramphenicol) or tetracycline. If you feed your fish frozen foods or chopped foods, try to use the same ratio with mixing. As a last resort add at most 10 mg per liter of water. Also, if unkempt conditions are the suspected cause, correct it.

Fish Vibriosis

  • Symptoms: Lethargy, increased respiration, loss of appetite, skin hemorrhages, and death.

Vibrio is a genus of Gram-negative bacteria found primarily in saltwater or brackish water, and consisting of 70 or more strains. Fish Vibriosis involves a variety of infectious strains of Vibrio bacteria, most notably Vibrio anguillarum, V. ordalii, V. damsela, and V. salmonicida.

Fish Vibriosis occurs most often in marine animals or brackish water fish, though it can occasionally be found in tropical species. Fish contract the bacteria through open sores or feeding on dead fish that died from the disease. Hemorrhaging starts with reddening or blood streaks under the skin surface, becoming red spots on the ventral and lateral areas of the fish. Swollen dark lesions develop, turning into ulcers and release bloody pus. There may also be eye problems with cloudy eye, which can lead to pop-eye and eye loss.

The course of a vibriosis infection in fish is usually very rapid. Most infected fish die without showing more visual signs than the ulcers, and sometimes death may occur suddenly before any signs are noticed at all.

The best treatment includes oral antibiotics. Kanamycin is one of the best, also chloramphenicol or furazolidone are good. When treating with antibiotics, it must be done in a quarantine tank rather than the main aquarium. This is because antibiotics will damage the biological filter in the main tank, throwing the nitrification cycle into reverse and cause a spike in nitrites and ammonia after just a few days.

A word of caution: People can become infected by Vibrio bacteria when handling infected fish. Hence the nicknames of “fish handler’s disease” and “aquarium handler’s disease”. People can become infected when water containing the bacteria comes into contact with cuts or open sores on the skin. The bacteria may be in swimming pools, aquariums, or coastal waters.

Vibrio infections are vary rare in the United States. Cholera is probably the most well-known illness caused by one of the vibrio bacteria. People sick with cholera usually have intestinal symptoms such as diarrhea and vomiting. Those with compromised immune systems can be at higher risk, even death, with some strains of Vibrio bacteria.

goldfish with ich disease
Image Credit: Zay Nyi Nyi, Shutterstock

Protozoan Diseases

Velvet, Rust – Gold Dust Disease (this is either Oodinium pilularis or Oodinium limneticum)

  • Symptoms: Peppery coating giving a yellow to light brown “dust” on body, clamped fins, respiratory distress (breathing hard as seen as frequent or quick gill movements), cloudiness of eyes, and glancing off decor or substrate, possible weight loss.

Velvet disease in freshwater fish is caused by either Oodinium pilularis or Oodinium limneticum, which are parasitic skin flagellates. This parasite swims in the aquarium until it finds a fish host and adheres to it.

  • In Oodinium pilularis (as well as with “Ich” Ichthyophthirius multifiliis) it eats into the cells of the epithelial layer of the skin and fins as well as through the mucous membrane in the mouth. The mature parasite then leaves the host and drops to the bottom of the aquarium or plants. It then forms a cyst that divides, forming between 34 – 64 new cells, then bursts freeing the new cells into the aquarium to find a fish host.
  • Oodinium limneticum is similar, but attacks the fish’s skin and fins rather than burrowing under the epithelial layer, so it is localized right on the surface. It also multiplies on the host rather than at the bottom of the aquarium or on the plants.

This disease has the appearance of a golden or brownish dust over the fins and body. The fish may show signs of irritation, like glancing off aquarium decor, shortage of breath (fish-wise), and clamping of the fins. The gills are usually the first thing affected. Velvet affects different species in different ways. Danios seem to be the most susceptible, but often show no discomfort. This disease is highly contagious and fatal.

They can be treated either in the separate or in the main tan. A good treatment is with copper sulphate at 0.2 mg per liter (0.2 ppm) to be repeated once in a few days if necessary. Aquarisol is one medication of this sort that is usually readily available at pet stores. Acriflavine (trypaflavine) may be used instead at 0.2% solution (1 ml per liter). There are things to be aware of with each of these treatments however. Acriflavine can possibly sterilize fish and copper can lead to poisoning, so the water should be gradually changed after a cure has been effected.

Marine Velvet – Velvet Disease (Oodinium ocellatum) (Syn: Amyloodinium ocellatum or Branchiophilus maris). Also called Coral Reef Fish Disease.

  • Symptoms: White, yellow to light brown, or grey “dusty” appearance on body, respiratory distress (fast breathing – gills opening more than 80 times per minute), loss of appetite, rubbing or scratching against decor or substrate.

Marine velvet is one of the most common maladies experienced in the marine aquarium, with the other being Marine Ich. It is found in all the oceans of the world and often infects wild and newly caught marine fish. Velvet disease in saltwater fish is caused by Oodinium ocellatum, (syn: Amyloodinium ocellatum or Branchiophilus maris), which is a parasitic skin flagellate. It is a fast moving disease that can cause mass casualties. Primarily it infects the gills of fish but can attach itself to the body as well, burrowing deep into the skin’s subcutaneous layer. Deaths are generally a result of interference to the respiratory system. This disease is highly contagious and fatal.

Chemical treatments for this disease include using copper (copper sulphate). Follow the instructions provided by the manufacturer. Generally it involves maintaining copper levels between .12 to .18 mg/L for 21 days, and using a quarantine tank is best. Natural methods include hyposalinity in a quarantine tank with a low salinity, by lowering salt level to around 1.009 – 1.010 for 10 days. A danger with with using low salinity is in re-acclimating the fish to a higher salinity. You must be able to accurately measure the salinity and must increase it very slowly.

Brooklynellosis – Clownfish Disease (Brooklynella hostilis)

  • Symptoms: loss of appetite, develops lesions on the body, sloughing off excessing body slime, heavy respiration/gasping for oxygen,constantly scratches against the substrate.

   Brooklynellosis is caused by a protozoan parasite called Brooklynella hostilis. This protozoan disease is mostly associated with clownfish, although other fish can contract it. This quick killer is a parasite that infests the gills and skin, where the fish develops lesions on the body, sloughing off excessing body slime and constantly scratches against the substrate. The infesting of the gills causes them heavy respiration as they gasp for oxygen. The fish will succumb to this disease in about 30 hours after the infection is identified. The first sign is a lack of appetite. Some newer strains of White Spot Disease or Crypt look similar to Brooklynella.

   Formalin and Malachite green are two active ingredients found in medications that have been used to treat this infection. One is a dip with 10 times the dose and one is as directed on the bottle to treat the entire quarantine tank. Hyposalinity which is 1.009 for the duration of treatment along Formalin is recommended. Copper has no affect on Brooklynella.


  • Symptoms: Milky cloudiness on skin.

   This is a rare protozoan disease that causes a cloudiness of the skin. The best treatment is with copper at 0.2 mg per liter (0.2 ppm) to be repeated once in a few days if necessary. Acriflavine (trypaflavine) may be used instead at 0.2% solution (1 ml per liter). As acriflavine can possibly sterilize fish and copper can lead to poisoning, the water should be gradually changed after a cure has been effected.
     Raising the water temperature to 80° – 83° F for a few days has also been effective.


  • Symptoms: The first symptom of slimy, white mucous feces, even while still eating and acting normal. Further signs are the fish hiding in the corner with it’s head down, the head above the eyes gets thin, they blacken in color, and swim backwards.

     Hexamita are intestinal flagellated protozoa that attack the lower intestine. Discus and other large cichlids, especially Oscars, are especially prone to Hexamita. Saltwater fish are affected on rare occasions as well As it is a disease of the digestive tract, a wasting away or loss of appetite may be experienced.

     An effective treatment is the drug metronidazole. Metronidazole is an antibiotic for anaerobic bacteria with anti-protozoal properties. This drug is reef safe, and medications are either added to the water or mixed with the fish food. Metronidazole works by ceasing the growth of bacteria and protozoa. Metronidazole is an antibiotic for anaerobic bacteria with anti-protozoal properties, and this drug is reef safe. Some available products that contain metronidazole include Seachem Metronidazole, Seachem AquaZole, Thomas Laboratories’ Fish Zole and National Fish Pharmaceutical’s Metro-Pro.
   A combined treatment in the food (1% in any food the fish will eat) and in the water (12 mg per liter) is recommended. Repeat the water treatment every other day for three treatments. And effective treatment is with Seachem’s Metronidazole.

     (This disease is often confused with another disease called Head and Lateral Line Erosion (HLLE), which use to be called “hole-in-the-head” disease, because both these diseases are often seen simultaneously in the same fish. Head and Lateral Line Erosion disease looks like cavities or pits on the head and face. This is not a protozoan disease, but is actually caused by environmental conditions.).

Ich – Ick – White Spot Disease (Ichthyophthirius multifiliis)

  • Symptoms: Salt-like specks on the body/fins.  Excessive slime. Problems breathing (ich invades the gills), clamped fins, loss of appetite.

   Ich, Ick, or White Spot Disease is the most common malady experienced in the home aquarium. Luckily, this disease is also easily cured if caught in time! Ich is actually a protozoa called Ichthyophthirius multifiliis. There are three phases to the life cycle of this protozoa. Normally, to the amateur aquarist, the life cycle is of no importance. However, since Ich is susceptible to treatment at only one stage of the life cycle, an awareness of the life cycle is important.

  • Adult phase – it is embedded in the skin or gills of the fish, causing irritation (with the fish showing signs of irritation) and the appearance of small white nodules. As the parasite grows it feeds on red blood cells and skin cells. After a few days it bores itself out of the fish and falls to the bottom of the aquarium.
  • Cyst phase – after falling to the bottom, the adult parasite forms into a cyst with rapid cell divisions occurring.
  • Free swimming phase – after the cyst phase, about 1000 free swimming young swim upwards looking for a host. If a host is not found within 2 to 3 days, the parasite dies. Once a host is found the whole cycle begins anew.

   These three phases take about 4 weeks at 70° F but only 5 days at 80° F. For this reason it is recommended that the aquarium water be raised to about 80° for the duration of the treatment. If the fish can stand it, raise the temperature even higher up to 85°.

     The free swimming phase is the best time to treat with chemicals. Raising the aquarium temperature to 80° F will greatly shorten the time for the free swimming phase to occur. The drug of choice is quinine hydrochloride at 30 mg per liter (1 in 30,000). Quinine sulphate can be used if the hydrochloride is not available. The water may cloud but this will disappear. By reducing the time (with raised temperature) of the phases, you should be able to attack the free swimming phase effectively.

     Some aquarists like to use malachite green, but it tends to stain the plastic and silicone in the aquarium. Most commercial remedies contain malachite green and/or copper, which are both effective.

Marine Ich – Crypt – Marine White Spot Disease (Cryptocaryon irritans)

  • Symptoms: Small white spots, nodules, or patches on their fins, body, or gills. Fish may produce excessive slime, show problems breathing (ich invades the gills), have frayed fins, loss of appetite, and cloudy eyes. White spots may not be obvious on light-colored fish or if the infection is just in the gills, however that may not rule out infection. Other indications can include rubbing or scratching against decor or substrate, abnormal swimming, hanging at the surface or on the bottom, acting lethargic, or breathing more rapidly as if in distress.

   Marine Ich, Crypt, or Marine White Spot Disease is one of the most common maladies experienced in the marine aquarium, with the other being Marine Velvet. It can grow in environments with excessive stress, poor water quality and fluctuations in water temperature. It can also come into the aquarium on a new fish that is a carrier.
   This protozoa has four phases to its life, lasting up to 38 days depending on the temperature of the environment. This parasite affects marine and brackish fish. Aquarists are most familiar with the stage where the protozoa is infesting the host, with small white spots on the fish’s body and fins. Unfortunately this visual clue is also the reason for difficulty in eradicating marine ich. Once the parasite has left the host’s body many aquarists believe their fish is cured and the problem is solved and so they cease treatment, only to have another larger reoccurrence.

   For eradication treatment must be carried through to completion, so understanding the parasite’s life cycle will greatly increase your chances of success. The life cycle is outlined here:

  • Trophont phase – when the parasite is growing in the skin or gills of the fish it appears as small white nodules, and the fish begins showing signs of irritation. It will spend 5 to 7 days (depending on the temperature) feeding on the fish. Once it reaches maturity it leaves the fish, reportedly after the lights go out. It is now called a protomont.
  • Protomont phase – the protomont will free swim or will crawl about the substrate for several hours (2 to 18 hours) producing a sticky wall around itself with which it is able to adhere to a surface. Once it adheres it begins to turn into a cyst and is now called a tomont.
  • Tomont phase – at this stage there is rapid cell divisions occurring, resulting in hundreds of daughter parasites that are called tomites. This stage can last anywhere from 3 to 28 days. Eventually the tomites hatch and begin swimming about looking for a host and are now called theronts.
  • Theront phase – newly hatched, they are swimming about looking for a host which they must find within 24 hours or they will die. Once a host is found they turn into trophonts and the whole cycle begins anew.

   The life cycle of this parasite can vary dramatically and is dependent on temperature. Optimal growth of most strains of Cryptocaryon appear to be about 73.4–86°F (23–30°C), and they cycle faster in a warmer environment. A common mistake is to confuse the treatment of this protozoan with the treatment for freshwater Ich (Ichthyophthirius multifiliis). Raising the temperature of the tank does not eliminate this protozoa like it does with freshwater Ich.
   Ideally the parasite would be eliminated while on the host or shortly after leaving the host. However, those that are buried in the gills are immune to treatment until they leave the fish. This along with the variability of the cycle makes it difficult to treat in such a timely manner.
   To rid the aquarium of this protozoa, it is recommended that you use a combination of water changes and chemical treatment, a multiple number of times.  A common mistake is to confuse the treatment of this protozoan with the treatment for freshwater Ich (Ichthyophthirius multifiliis). Raising the temperature of the tank does not eliminate this protozoa like it does with freshwater Ich.

  • Chemical Treatments:
    Chemical treatments for this disease include using copper, formalin, or a combination of copper and formalin. Follow the instructions provided by the manufacturer. Achilles Tangs are sensitive to cooper based products so use them carefully. Equally effective is a (normal saltwater) short “dip” with Formalin (see directions on bottle for the dip) for these patients, and remove them to a separate tank.
  • Natural Treatments:
    Natural methods include either a quarantine tank with a low salinity (hyposalinity), dips, or large frequent water changes.
    Low Salinity Method: For low salinity keep the specific gravity of the water at approximately 1.009-1.010 with temperatures of 78 – 80° F (25 – 27° C) for 14 days. A problem with this method are that lower salinities are not as easily tolerated by many common tropical marine species, including angelfish and deep water fish. These types of fish have a much shorter time that they can tolerate the lower salinity. A danger with with using low salinity is in re-acclimating the fish to a higher salinity. You must be able to accurately measure the salinity and must increase it very slowly.
    Dip Method: Using freshwater or or lower salinity dips, either with a duration of just a few minutes, or short to prolonged immersion baths (duration in hours to days) can be used for tolerant fish to kill or reduce the numbers of external parasites on the fish. Unfortunately trophonts and tomonts are more protected, so longer dips and baths are needed than for other parasite species.
    Water Change Method: For the water change method, replace 50% of the aquarium water daily for 14 days. This is perfectly safe method as long as temperature and salinity are the same, and this will remove the parasites while in a free swimming stage.

   Reportably some healthy fish can develop a limited immunity. It may not be a total immunity, rather being just a small amount of infestation rather than extensive infestation. This immunity is short-lived lasting only about six months.

Neon Tetra Disease

Symptoms: Whitened areas deep into the fishes’ flesh. Muscle degeneration leading to abnormal swimming movements.

   So named for the fish it was first recognized on, the Neon Tetra. It is caused by the sporozoa Plistophora hyphessobryconis. Even though it is named after Neon Tetras, it can appear on other fish. Whitish patches appear as if just below the skin. In Neon Tetras it destroys the bright blue-green neon stripe. The organisms form cysts which burst and release spores. The spores penetrate further and form more cysts. Eventually, the spores migrate to the water and are eaten by other fish in the food. These spores migrate into the digestive tract, then the muscles, and a new infection starts.
     There is no known cure. It is best to destroy the infected fish and clean the aquarium.

Glugea and Henneguya
    Symptoms: Similar to Lymphocystis, the fish will have nodular white swellings on fins or body.

     Glugea and Henneguya are sporozoans that form large cysts on the fish’s body and release spores. Luckily, these diseases are very rare. The fish bloat up, with tumor-like protrusions, and eventually die.
     No cure, as of yet. It is best to destroy the infected fish before the spores can spread.

    Symptoms: Dulling of the colors due to excessive slime, fraying of the fins, weakness, gill damage 

     This disease causes a blue white cloudiness on the skin and attacks the gills. Later the skin may be broken down and the gills destroyed. The fish may behave like they have irritations, by glancing off aquarium decor, they may have clamped fins and difficulty breathing.
     Acriflavine (trypaflavine) may be used at 1% solution (5 ml per liter). As acriflavine can sterilize fish, the water should be gradually changed after a cure has been effected. It also helps to raise the temperature to about 80° F.

African Bloat – “Malawi Bloat”
    Symptoms: The first sign of ‘bloat’ is loss of appetite which is then followed by swelling of the abdomen, labored breathing, listlessness, reclusiveness, possible red striations on the body, and stringy white feces.

   There seems to be no explainable rationale as to its cause of bloat. Once a fish becomes afflicted it is often fatal. A fish that is not eating must be treated immediately or it can quickly become incurable and die. Though It is not certain what this disease is, it is generally believed to be caused by a protozoal parasite complicated by bacterial infection. Bloat is a serious malady often associated with African cichlids especially those from Lake Malawi, thus the common name ‘Malawi Bloat’. The Tropheus species from Lake Tanganyika are also very susceptible.

   The most common cause of this disease is stress and the first sign if illness is not eating. Stress can be caused by such things as transport, netting, poor water quality, insufficient diet, over feeding, and a lack of hiding places. Other causes, that are easily remedied, are an improper diet and adding too much salt to the water. Prevention is of utmost importance, and It is possibly to cure a fish if treated right away.

   Following are some techniques aquarists use:

  • Any new specimens you obtain can have bloat or will often soon develop it. When you first acquire them try to provide them with the same food that the dealer was feeding, and then wean them onto a good vegetable based diet; Spirulina flake and pellet.
  • Some will soak the food in dissolved metronidazol and feed them that for the first few days when first obtained. Seachem makes a metronidazol that can be bound to food when used with their Focus product.
  • A good vegetable based diet is important.
  • A healthy group of fish will eat with gusto. But even though they can be very active feeders it is important to not overfeed them. Keep an eye on them, and if one is not eating with vigor some aquarists will then treat the tank with Clout.
  • One author says that they will segregate an ailing fish the second they see signs of not eating, and then will do water changes every day for 5 days in the main aquarium.

   Metronidazol is considered the most reliable cure and some use Clout as another cure, but do not use them together.

betta fish with fungal disease
Image Credit: MyBabesea, Shutterstock

Parasitic Diseases

 Fish louse (Argulus)
    Symptoms: The fish scrapes itself against objects, clamped fins, visible parasites about 1/4 inch in diameter are visible on the body of the fish.

   The fish louse is a flattened mite-like crustacean about 5 mm long that attaches itself to the body of fish. They irritate the host fish which may have clamped fins, become restless, and may show inflamed areas where the lice have been.
     With larger fish and light infestations, the lice can be picked off with a pair of forceps. Other cases can best be done with a 10 to 30 minute bath in 10 mg per liter of potassium permanganate. Or treat the whole tank with 2 mg per liter, but this method is messy and dyes the water.

Anchor Worm (Lernaea)
    Symptoms: The fish scrapes itself against objects, whitish-green threads hang out of the fish’s skin with an inflamed area at the point of attachment.

   Anchor worms are actually crustaceans. The young are free swimming and borrow into the skin, go into the muscles and develop for several months before showing. They release eggs and die. The holes left behind are ugly and may become infected.
     The anchor worm is too deeply imbedded to safely remove. Treatment can best be done with a 10 to 30 minute bath in 10 mg per liter of potassium permanganate. Or treat the whole tank with 2 mg per liter, but this method is messy and dyes the water.

Black Spot – Black Ick (diplopstomiasis)
    Symptoms: The fish, very irritated, scrapes itself against objects, appears as small black specks or smudges on the body and around the mouth, and if heavily infected may experience blood loss.

   Black Spot or Black Ick is rare in aquariums. It is generally seen in outdoor ponds, especially those with mud bottoms, but it can be introduced when adding new fish into the aquarium. Fish that are most readily susceptible are the Silver Dollar, Piranha, or other fish of these types. In general it does relatively little damage to the fish, even if they are heavily infested.

   The disease is caused by a parasite (larval trematode) that burrows into the skin of a fish where it forms a cyst that is about one millimeter in diameter. It has a complex lifecyle that in order to survive, requires fish eating birds or animals, snails, or fish at different stages infected with the disease.

   Black Spot is generally easy to cure. Either treat with salt baths or there are a number of commercially available treatments and preventatives.

    Symptoms: The fish scrapes itself against objects, whitish-green threads hang out of the fish’s gills.

   This parasite is like the anchor worm, but is smaller and attacks the gills instead of the skin. Treatment can best be done with a 10 to 30 minute bath in 10 mg per liter of potassium permanganate. Or treat the whole tank with 2 mg per liter, but this method is messy and dyes the water.

    Symptoms: The fish scrapes itself against objects, rapid gill movement, mucus covering the gills or body, the gills or fins may be eaten away, the skin may become reddened.

   There are many species of flukes, which are flatworms about 1 mm long, and several symptoms that are visible. They infest gills and skin much like ich, but the difference can be seen with a hand lens. You should be able to see movement and possibly eye spots, which is not found in ich. Gill flukes will eventually destroy the gills thus killing the fish. Symptoms of a heavy infestations are pale fish with drooping fins, rapid respiration, glancing off aquarium decor, and /or hollow bellies.
     Treatment can best be done with a 10 to 30 minute bath in 10 mg per liter of potassium permanganate. Or treat the whole tank with 2 mg per liter, but this method is messy and dyes the water.

    Symptoms: Worms hanging from the anus.

   Nematodes (threadworms) infect just about anywhere in the body but only shows itself when they hang out of the anus. A heavy infestation causes hollow bellies. Lighter infestations usually cause no problems with the fish.
     Short of destroying the fish, which is easier, two treatments have been suggested. First treatment; soak the food in parachlorometaxylenol and give the fish a bath or treat the aquarium with 10 ml per liter. The bath should last for several days. Second treatment; find special food containing thiabendazole as a nematode (threadworm) cure and hope the fish will eat it.

    Symptoms: Leeches are visible on the fish’s skin.

   Leeches are external parasites and affix themselves on the body, fins, or gills of the fish. Usually they appear as heart shaped worms (they are just curled up) attached to the fish. They are usually introduced to the aquarium via plants or snails.
     Since leeches are sucking and borrowing into the surface of the fish, removal with forceps can cause great damage, if not death, to the fish. If the fish is bathed in a 2.5 percent solution of salt for 15 minutes, most of the leeches should just fall off. Those that do not will be affected enough to remove with forceps with minimal damage. Another treatment is to add Trichlorofon at 0.25 mg/l to the aquarium. Live plants should be removed and treated with potassium permanganate at 5 mg/l before replanting.

Uronema marinum
    Symptoms: Skin scraping, pale discoloration, loss of color, weight loss, dehydration, flashing, and rapid breathing.

   The saltwater parasite, Uronema marinum, is a free-living ciliated protozoa that can cause fatal infections in marine fish. It is an opportunistic feeder that normally eats on bacteria, but when the immunization of a fish is low it will attack, invading the fish’s muscles and internal organs. This infestation is often the result of the introduction of a new fish, overcrowding, and poor water quality resulting from a high organic load in the aquarium.
   This parasite is difficult to identify as the symptoms can also be indicative of other parasitic and bacterial problems. However, it can be debilitating and ultimately fatal to a variety of marine fish including Tangs, especially the Yellow Tang, Angelfish species especially those in the genus Centropyges, Seahorses, many species of Butterflyfish, yellow headed Jawfish, and others.

   The best way to avoid the problem is to keep your current tank free from infestation. Quarantine all new fish for a period of three weeks, improve the water quality of the tank, and reduce the stress level in the aquarium by reducing the number of fish and incorporating places for fish to hide and rest.

   There are several types of medications that can be used to treat infected fish:

  • Medications such as Malachite green, Copper Sulfate, or Methylene blue. Use caution and be sure to follow the manufacturers instructions.
  • Freshwater bath – place infected fish in the freshwater bath for a period of three minutes or until the fish shows signs of stress.
  • Low salinity (hypo salinity) treatment – lower the salinity in the quarantine tank to a specific gravity of 1.011 and maintain at this salinity for 21 days. Do not use this treatment with invertebrates or especially sensitive fish such as sharks and rays.
  • Nitrofurazone – an antibiotic that has some antiparasitic action, and can be helpful when used along with formalin dips.

Fungal Diseases

Fungus (Saprolegnia)
    Symptoms: Tufts of dirty, cotton-like growth on the skin, can cover large areas of the fish, fish eggs turn white.

   Fungal attacks always follow some other health problem like parasitic attack, injury, or bacterial infection. The symptoms are a gray or whitish growth in and on the skin and/or fins of the fish. Eventually, if left untreated, these growths will become cottony looking. The fungus, if left untreated, will eventually eat away on the fish until it finally dies.
     After ascertaining the initial cause of the fungus and remedying that, use a solution of phenoxethol at 1% in distilled water. Add 10 ml of this solution per liter of aquarium water. Repeat after a few days if needed, but only once more as three treatments could be dangerous to aquarium inhabitants. If the symptoms are severe the fish can be removed from the aquarium and swabbed with a cloth that has been treated with small amounts of povidone iodine or mercurochrome.

   For attacks on fish eggs, most breeders will use a solution of methylene blue adding 3 to 5 mg/l as a preventative measure after the eggs are laid.

    Symptoms: Sluggishness, loss of balance, hollow belly, external cysts and sores.

    Ichthyosporidium is a fungus, but it manifests itself internally. It primarily attacks the liver and kidneys, but it spreads everywhere else. The symptoms vary. The fish may become sluggish, lose balance, show hollow bellies, and eventually show external cysts or sores. By then it is usually too late for the fish.
     Treatment is difficult. Phenoxethol added to food as a 1% solution may be effective. Chloromycetin added to the food has also been effective. But both of these treatments, if not watched with caution, could pose a risk to your fish. It is best, if diagnosed soon enough, to destroy the affected fish before the disease can spread.


Head and Lateral Line Erosion Disease – Hole-in-the-Head Disease (HLLD or HLLE)
   Symptoms: Begins as small pits on the head and face, usually just above the eye. If untreated, these turn into large cavities and then the disease progresses along the lateral line.

   Head and Lateral Line Disease is also known as Hole-in-the-Head Disease, Lateral Line Erosion (LLE), and Lateral Line Disease (LLD). In saltwater fish it is occasionally referred to as Marine Head and Lateral Line Erosion (MHLLE) or Head and Lateral Line Erosion Syndrome (HLLES).

   Though its cause is not definitively determined, a recent study was conducted by Jay Hemdal and reported in Coral Magazine in the spring of 2011. The focus of the study was to evaluate the relationship between the use of activated carbon in aquariums and the development of HLLES in surgeonfish. From the results of the study, it has been suggested that HLLE is a result of activated carbon used in the aquarium. Fish from the study that developed HLLE were in two control groups, one group treated with unwashed lignin carbon and the other with pelletized carbon. A third group of fishes were in a control group where no carbon was used, and they did not develop Head and Lateral Line Disease.
  The study was conducted only on marine fishes. Not all species of fish show the same symptoms of the disease however, and they do not always develop lesions to the same degree. It has been suggested that in freshwater fishes the causes seem to be different, but that is not yet substantiated.

   Previously Head and Lateral Line Disease was thought to be caused by a poor diet or lack of variety, a nutritional deficiency of one or more of: Vitamin C, Vitamin D, calcium, and phosphorus; lack of partial water changes; or over filtration with chemical media such as activated carbon.
   The best treatment suggestions at this time are to use a quarantine tank that offers a stress free environment with good quality water. Provide a quality diet including vegetable foods, places to hide, and a quiet area for the aquarium.

   HLLE has been reversed by one or more of the following treatments:

  • Remove activated carbon filtration
  • Increase frequent water changes.
  • Add vitamins to frozen foods.
  • Add the addition of flake foods, as they are enriched with vitamins.
  • Add greens, either frozen or in leaf form, to the diet.
  • Decrease the amount of beef heart as it lacks many critical nutrients.

   (This disease is often confused with another disease called ‘Hexamita‘, because both these diseases are often seen simultaneously in the same fish. Hexamita is a protozoan disease that attacks the lower intestine. Discus and other large cichlids, especially Oscars, are especially prone to Hexamita.)

Eye Problems
   Symptoms: Cloudy cornea, opaque lens, pop eye, swelling, blindness.

  • Cloudy cornea can result from a bacterial invasion. Antibiotics may help.
  • Opaqueness can result from poor nutrition or a metacercaria invasion (grubs). Try foods with added vitamins and changing the diet to include variety.
  • Pop eye (exophtalmia) can result from rough handling, gas embolism, tumors, bacterial infection, or vitamin A deficiency. Gas bubble or bacterial infection can be treated successfully with penicillin or amoxicillin.
  • Blindness can be caused by poor nutrition or excessive light. Lowering the light level and a change of diet to include lots of variety may help prevent it.

Swim-bladder Disease
   Symptoms: Abnormal swimming pattern, difficulty maintaining equilibrium.

   Swim bladder problems usually indicate another problem listed here. If you suspect swim-bladder problems in a fish, first check and treat it for other diseases as listed below:

   If you have eliminated other causes, make sure you are feeding the right food and make sure the fish is not constipated. Give it live food for awhile to ensure it is getting enough roughage. Also, check the temperature for your fish’s requirements and keep the temperature stable.

Non-infectious Maladies

Physical Injuries
     Even in the best of aquariums under the supervision of the most astute aquarists, injuries occur. Some times a bully fish is the culprit, or sharp decor. Sometimes there appears to be no explanation. As in the human world, accidents happen.
     If the cause of the injury is obvious, it should be remedied. Then the injury should be treated. The injury should be touched with 2% Mercurochrome, which is supplied commercially. Also, depending on the fish’s tolerance to water conditions, keeping the fish in slightly acid water should speed recovery (pH 6.6). Minor injuries, if the water conditions are good, should just heal themselves.

     Some fish are more susceptible to constipation than others. Usually fish with more compressed bodies like angelfish and silver dollars. Symptoms are loss of appetite and swelling of the body. The cause is almost always diet.
     Usually, with a change of diet, the condition rights itself. But in stubborn cases try dried food that has been soaked in medicinal paraffin oil. Glycerol or castor oil may also be used. If the diet is changed on a regular basis and live foods offered occasionally this condition may never occur.

     Tumors can be caused by a virus or a cancer, but most tumors are genetic. The genetic tumors may be caused from too much hybridization, common amongst professional breeders.
   Practically all tumors are untreatable. If the fish is in distress, it should be destroyed.

Congenital Abnormalities
     Abnormalities usually occur when professional breeders are trying to acquire certain strains in breeds. Most are beneficial abnormalities like albinism or extra finnage. But undesirable abnormalities crop up and are usually culled out by the breeder. However, such abnormalities sometimes happen in the amateur aquarium.
     If the abnormality is not life-threatening or degrades the quality of life, just leave it be and brag to your friends about the unusual inhabitant. Otherwise, the fish should be humanely destroyed.

Viral Diseases

    Symptoms: Nodular white swellings (cauliflower) on fins or body.

    Lymphocystis is a virus, and being a virus, it affects the cells of the fish. It usually manifests itself as abnormally large white lumps (cauliflower) on the fins or other parts of the body. It can be infectious, but is usually not fatal. Unfortunately there is no cure, but fortunately this is a rare disease.

     There are two suggested treatments. One treatment is to remove and destroy the infected fish as soon as possible. The other treatment is to simply separate the infected fish for several months and hope for remission, which usually does occur.

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