Glass Knife Fish

The Glass Knifefish has a peaceful, shy demeanor and does best kept in the company of its peers!

The Glass Knifefish Eigenmannia virescens are unusual, even for knife fish. The fins and much of the body are transparent and they have a most interesting swimming motion. Their bodies are relatively stiff and they don’t have a dorsal fin, so Its hard to imagine how they can move around so adeptly. When they move they appear to be jet propelled. Its that hard to see anal fin using a rippling motion that provides their means of locomotion. If they are going forward and want to reverse direction, they simply reverse the rippling motion and are moving backward.

This transparent knife fish is a member of the Sternopygidae Family found in tropical South America. As a group they are referred to as Glass knifefish or Rattail Knifefish. There are currently 30 species of glass knifefish grouped into six genera. The Eigenmannia genus has eight described species including E. virescens.

There are a couple of distinguishing features of these Transparent Knifefish besides having a rather stiff, semi-transparent body and transparent fins. The body is shaped like the blade of a knife, hence the common name of Glass Knifefish. They have a snout that is relatively short rather than the tubular or elongated snout seen on many other types of knifefish, and they have relatively large eyes. Sometimes the body is colored with a very slight green tint, and so it is also called the Green Knifefish.

The Glass Knifefish are non-aggressive and are also quite timid. They do best when kept in a large species tank with their own kind. These are a fish that will do best in groups of 5 or more. They will develop a hierarchy of dominant and submissive fish, but they will not injure each other. They actually communicate using electrical pulses. These are one of the few Knife fish that thrive on the companionship of its own kind and it is an amazing sight seeing these fish together communicating.

Even with a small group however, they can appear to be shy. This is because they are naturally nocturnal so they are not usually active during daylight hours. Make they feel secure by giving them plenty of hiding places with plants and bogwood. Clear acrylic or plastic tubes also work well for hiding places. These fish have poor eyesight so they use electrical impulses as a sort of radar for navigation and locating objects. So even though the tube is clear, their radar tells them that they are in a secure place, but now the aquarist can see them.

• For information on keeping freshwater fish, see: Freshwater Aquarium Guide: Aquarium Setup and Care

Habitat: Distribution / Background

The Glass Knifefish Eigenmannia virescens was described by Valenciennes in 1836. They are found throughout the Amazon River basin in South America. Most specimens offered for sale are exported from Colombia. This species is not listed on the IUCN Red List. Other common names it is known by are Green Knifefish and Transparent Knifefish.

These fish inhabit freshwater flood plains, ponds and creeks where the substrate has lots of plant detritus. Many of these fish live their life in deep swift moving waters and they have adapted with a unique way of swimming vertically. In nature these fish eat crustaceans, insects and smaller fish.

• Scientific Name: Eigenmannia virescens
• Social Grouping: Groups
• IUCN Red List: NE – Not Evaluated or not listed

Description

The male Glass Knife can reach up to about 17 1/2 inches (45 cm), though females are smaller at not quite 8 inches ((20 cm). They have a lifespan of about 3 – 6 years. The semi-transparent body of the Glass Knife Fish, occasionally with a tint of green, is flat and elongated. They don’t have a dorsal fin or a caudal fin, but there is a continuous fin along the underside that moves with an undulating motion. It can be difficult to see this rippling anal fin which provides their means of locomotion. Because their bodies are relatively stiff and their fins and much of their bodies are transparent, they appear to be jet propelled.

This fish has an electric organ which allows it to generate a very weak electric field around its body. The field helps with a variety of things such as identifying objects in the water, gives it spatial orientation, helps it to navigate, and helps it identify food. Males use an electric ‘stereotyped’ communication to court females.

• Size of fish – inches: 17.7 inches (45.01 cm) – The maximum size for a female is about 8 inches (20 cm) and males are said to grow to as large as 17 inches (45 cm).
• Lifespan: 6 years – They have a lifespan of about 3 – 6 years.

Fish Keeping Difficulty

The Glass Knifefish is suggested for an aquarist with some fish keeping experience. They are very hardy after the first 30 days in a new environment, but there is a high mortality rate initially. These fish are mainly wild caught and can be very timid and hard to fed at first. Wild caught specimens can carry parasites. Because they do not have scales they are prone to disease and sensitive to medications. A UV sterilizer is very effective in preventative health of your knife fish. Once you get past the first 30 days they are very easy to take care of.

• Aquarium Hardiness: Moderately Difficult
• Aquarist Experience Level: Intermediate

Foods and Feeding

The Glass Knife Fish are carnivores. In the wild they are nocturnal, when the sun sets and throughout the night they feed on eat crustaceans, insects and smaller fish. In the aquarium their feed should consist of smaller sized fresh or frozen meaty foods such as bloodworms, brine shrimp or blackworms, tubifex, small crustaceans, snails, and small fishes. Freeze dried foods will sometimes be accepted. They are shy and normally only eat at night unless trained to eat during the day, which can be done.

• Diet Type: Carnivore
• Flake Food: Occasionally
• Tablet / Pellet: Occasionally
• Live foods (fishes, shrimps, worms): Most of Diet
• Meaty Food: All of Diet
• Feeding Frequency: Daily

Aquarium Care

This fish is scaleless and very sensitive to water condition changes. A high quality filter is a must. Weekly water changes of 30 – 50% are suggested. Water condition tests should also be done weekly to make sure levels are not spiking.

• Water Changes: Weekly – Do a 30 – 50% water change weekly.

Aquarium Setup

The Glass Knife Fish will generally stay near the bottom where there is cover for them to hide and feel safe. An aquarium of around 55 gallons will work when they are young, but you will need to eventually provide a tank of 100 gallons or more if you plan on keeping these beauties into adulthood. Use a high quality filter with low water flow. A UV sterilizer is a smart thing to incorporate into your tank as these fish are very sensitive to medications. The UV sterilizer will kill many diseases.

These fish thrive in large, well-planted aquariums decorated with a variety of hiding places within rockwork and plants. Clay flower pots make great caves for these fish so they have a safe place to take refuge. Some aquarists will use a clear tube for the fish to hide in. This makes it feel secure, but also the aquarist can see it. Soft sand is the best choice for substrate.

They are nocturnal and will spend most of the daylight hours hiding in a safe and secure location. They prefer dim lights, so use more subdued lighting or offer cover with floating aquarium plants. Since these fish are very social amongst their own species, so it is best to keep more than one together in the aquarium. Once they become accustomed to living in an aquarium, they will become more outgoing, especially at feeding time.

• Minimum Tank Size: 55 gal (208 L) – A 55 gallon tank will work when they are young, but they will need a tank size of 100 gallons as adults.
• Suitable for Nano Tank: No
• Substrate Type: Sand
• Lighting Needs: Low – subdued lighting
• Temperature: 68.0 to 86.0° F (20.0 to 30.0&deg C) – Though they are tropical fish, they can be found in colder waters as low as 64.4° F (18° C).
• Range ph: 6.0-7.0
• Hardness Range: 2 – 15 dGH
• Brackish: No
• Water Movement: Weak
• Water Region: Bottom – Glass Knife Fish will generally stay near the bottom where there is cover for them to hide and feel safe.

Social Behaviors

The Glass Knifefish are peaceful and can be kept in a community tank with other peaceful fish of about the same size or larger. Most knifefish are known as stealthy predators with relatively large mouths but Glass Knifefish are an exception to this generalization. They are predators but their mouths are relatively small so they will not eat your other pet fish unless they are pretty small.

The Glass Knife Fish are timid and non-aggressive. Unlike most Knife fish, the Glass knife does well in groups of its own kind. They will do best in groups of 5 or more. They will develop a hierarchy of dominant and submissive fish, but they will not injure each other.

• Venomous: No
• Temperament: Peaceful
• Compatible with:
  • Same species – conspecifics: Yes – They are very social with their own kind, and do best in groups of 5 or more
  • Peaceful fish (): Monitor – While it is not necessarily aggressive, it will eat anything small enough to be considered a meal.
  • Semi-Aggressive (): Threat
  • Aggressive (): Threat
  • Large Semi-Aggressive (): Threat
  • Large Aggressive, Predatory (): Threat
  • Monitor
  • Shrimps, Crabs, Snails: May be aggressive – In the wild, this fish hunts at night for worms, crustaceans, insects and snails.
  • Plants: Safe

Sex: Sexual differences

Males grow to be much larger than the females.

Breeding / Reproduction

Glass Knifefish have reportedly been spawned in captivity although this is not a common occurrence. Practicalfishkeeping.co.uk reports that knifefish were spawned by using the following method: The water level in the tank was raised and the pH and water temperature were lowered to simulate a rainstorm. 100+ eggs are laid on floating plants where they are fertilized by the dominant male.

Breeding fish lay their eggs in plants. Once the fry are hatch it is important to separate the parents from the fry to assure that the fry will live. Breeding in the home aquarium can be difficult because one needs to stimulate the rainy season to encourage spawning. A good way to encourage spawning is to add cold water to the tank which will give the illusion of the raining season in which they normally breed.

• Ease of Breeding: Difficult

Fish Diseases

The Glass Knife does not have scales which make it more prone to disease. These fish are hardy and disease is not usually a problem in a well maintained aquarium. That being said there is no guarantee that you won’t have to deal with health problems or disease. Glass Knife are normally the first fish in a tank to show signs of ick and will twitch and rub around the tank. They respond well to most medication and normally heal quickly. NEVER use copper in a Glass Knife Fish tank.

Anything you add to your tank can bring disease to your tank. Not only other fish but plants, substrate, and decorations can harbor bacteria. Take great care and make sure to properly clean or quarantine anything that you add to an established tank so not to upset the balance. Because these fish eat live food, disease can be passed to them from their foods. Make sure to quarantine live food before feeding.

When keeping more sensitive types of fish, it is common for all fishes to be infected even before the first warning signs can be noticed. The best way to proactively prevent disease is to give your fish the proper environment and give them a well balanced diet. The closer to their natural habitat the less stress the fish will have, making them healthier and happy. A stressed fish is more likely to acquire disease.

As with most fish the Glass Knife are prone to skin flukes, parasitic infestations (protozoa, worms, etc.), ichthyobodo infection, parasitic infestations (protozoa, worms, etc.), bacterial infections (general), and bacterial disease. It is recommended to read up on the common tank diseases. Knowing the signs and catching and treating them early makes a huge difference. For information about freshwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments.

Availability

Glass Knife Fish are commonly available.

 Eigenmannia virescens (Image Credit: Ictiologia Universidad Católica de Oriente, Wikimedia Commons CC BY-SA 2.0 Generic)