The Black Spotted Eel may not be brightly colored but it has a beautiful design!

The Black Spotted Eel Mastacembelus dayi has an exotic-looking color pattern. As with most members of the spiny eel family Mastacembelidae, the attractiveness of this eel lies in its unusual color pattern and interesting behavior. consequently, this good-looking spiny eel is called the Polka Dot Eel and Spotted Spiny Eel as well. It is also one of the spiny eels occasionally referred and sometimes confused with the Tire Track EelMastacembelus favus and the Zig Zag EelMastacembelus armatus.

Spotted Spiny Eel adults have many spots running in horizontal lines along its entire length. Smaller specimens have fewer spots, but the spots are larger. This elegant patterning makes it a beautiful fish, one that is well worth having in your aquarium.

Although not considered to be true eels, the body shapes of all members of the spiny eel family are definitely eel-like. The Black Spotted Eel has an elongated with a long snout. It is a pretty good sized eel with imported specimens commonly being up to 20 inches (50 cm) in length. It will need an aquarium large enough to accommodate this length. A tank that is at least 36 inches long and about 35 gallons can house a younger specimen, but adults will need tanks that are 48 inches in length or longer and 55 gallons.

The Black Spotted Eel is a very attractive little fish and makes a nice show specimen in the aquarium. Spiny Eels are durable fish with a life span between 8 to 18 years. This spiny eel will tend to be rather shy fish when first acquired. It will get along well in a community tank but it will need places to retreat. It is peaceful with larger tank mates but not with smaller fish. Spiny eels are best kept singly however, as they will also generally fight with others of their own kind. Over time, as spiny eels become comfortable they become more secure. Some will even eventually take food from their keepers hand.

The Black Spotted Eel has been imported for a long time, and over the past few years many thousands have been shipped from Thailand. M. dayi has been considered to be very rare by science, but fortunately this is most likely not the true situation. This is probably just a case of local fisherman knowing more about the distribution of a fish than the scientists do. Supposedly only 3 specimens had ever been found initially, and these 3 were used to describe the species (Boulenger in 1912).

Hand-feeding the Black Spotted Eel


Black Spotted Eel – Quick Aquarium Care

Aquarist Experience Level:Intermediate
Aquarium Hardiness:Moderately hardy
Minimum Tank Size:75 gal (284 L)
Size of fish – inches20.0 inches (50.80 cm)
Temperature:74.0 to 82.0° F (23.3 to 27.8&deg C)


Habitat: Distribution / Background

The Black Spotted Eel Mastacembelus dayi was described by Boulenger in 1912. They are found in Asia from the Irrawady and Chindwin river drainages of western Myanmar, and eastern Thailand. This species is listed on the IUCN Red List as least concern (LC) because it is believed to be widely distributed and likely fairly common, although harvested by fisheries and for the aquarium trade, there is no significant threats identified at present. Other common names it is known by are the Polka Dot Eel and Spotted Spiny Eel. It is also one of the spiny eels occasionally referred to as a Tire Track Eel.

The spiny eels inhabit the moving waters of rivers and streams with sandy to pebbly bottoms, normally with dense with vegetation. They may also inhabit still waters and migrate during the dry season into canals, lakes and floodplain areas. They often bury themselves during the day in the gravel substrate, coming out at night to feed on insects larvae, worms, and vegetation.

  • Scientific Name: Mastacembelus dayi
  • Social Grouping: Solitary
  • IUCN Red List: LC – Least Concern


The body of the Black Spotted Eel is elongated with a long snout. Both the dorsal and anal fins are extended and joined to the caudal fin. These fish are commonly imported at over 20 inches (50.8 cm) in length. Spiny eels can live for 8-18 years when given proper care.

The background coloration is a dark tan to light brown. Along the length of its body it has a series of spots in a linear fashion. The spots are relatively large, with those on the upper part being dark and those lower down becoming paler. Some of the spots, especially along the center, can often become vertically elongated rather than round. This patterning extends from the tip of its mouth to the rear of the fish. Smaller specimens have fewer but larger spots.

  • Size of fish – inches: 20.0 inches (50.80 cm) – Fish of over 20 inches (50.8 cm) are commonly imported. There is some question over the maximum adult size of this fish, it is established at over 20 inches. How much it may exceed this is unclear.
  • Lifespan: 18 years – Spiny eels have a lifespan of 8-18 years.

Fish Keeping Difficulty

This eel is fairly easy to care for by an aquarist with some experience. They are normally great eaters and not very fussy. They do however require extremely pristine water. They have very small scales protecting their body so are prone to fungus and parasites and very sensitive to medications. These fish respond poorly to copper based medications, so these should be avoided. If cared for properly, these eels get fairly large and can live for a long time.

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

Foods and Feeding

In nature the Black Spotted Eel is omnivorous. They feed at night on benthic insect larvae, worms, and other aquatic invertebrates, but they will also eat some plant matter. Feeding is not a problem because they will eat most fresh or frozen foods including small fish, worms, and shrimp. They may be trained to eat freeze dried brine shrimp or bloodworms but this is not something that can be counted on. They will also eat small fishes so make sure their tank mates are too large to be able to fit into their mouths.

This fish is nocturnal and likes to be fed after the lights are turned off for the night. Eels only need to be fed a couple of times a week and some may refuse food offered more than that, then often eating only once every two or three weeks. The middle and top inhabitants will often eat all the food before it sinks.

  • Diet Type: Omnivore
  • Flake Food: Occasionally – They will eat flake but it is not enough for the eel to survive on.
  • Tablet / Pellet: Occasionally – Not all specimens will accept processed foods.
  • Live foods (fishes, shrimps, worms): Some of Diet
  • Vegetable Food: Some of Diet – Some Black Spotted Eels will accept vegetable matter, but this is fairly rare.
  • Meaty Food: Most of Diet
  • Feeding Frequency: Weekly – Eels only need to be fed a couple of times a week, and sometimes will eat even less.

Aquarium Care

The most important thing for these eels is that they always have clean and well-oxygenated water. Frequent water changes of about 30% a week are needed for this eel. With your weekly water change make sure to vacuum the gravel to remove all excess food and waster. but It’s best not to remove any bio film on rocks and decor. A magnet algae cleaner normally does a great job in keeping the viewing pane clear.

It is also helps to add efficient bottom cleaning tank mates to keep the bottom free from decaying foods in between cleanings. Be careful however, to add bottom cleaners after your eel is adjusted to its tank and is eating.

  • Water Changes: Weekly – Water changes of about 30% weekly.

Aquarium Setup

Black Spotted Eels will spend most of their time on the bottom of the aquarium. This species of spiny eel is commonly imported at over 20 inches (50.8 cm) in length, so plan accordingly. They will need a tank that 48 inches in length or longer and up to 75 gallons or more as they grow.

They do best in a soft to medium water with good water movement that provides plenty of oxygenation. These fish require pristine water. The tank water should turnover at least 10-15 times per hour. An undergravel filter is a great choice for these fish as it creates high oxygen through out the tank as well as reducing the waste. A canister filter or powerheads and airstones can be introduced to achieve proper flow and oxygenation. Provide a tight fitting lid as spiny eels are escape artists.

They like a dimly lit aquarium or one with floating plants to help subdue the light. If their tank has a sand or fine gravel substrate, they may burrow into it. Make sure they have plenty of hiding places so they will feel secure in their new home. Provide other decor such as rocks, caves, and roots to give it some dark areas to retreat. PCV tubing also makes great caves for long spiny eels. Be sure to place heavy decor firmly on the bottom. These fish are not actively destructive, but because of their size and burrowing nature, they can dislodge anything that gets in their way, including plants. Multiple hiding places need to be made for the eel to feel safe in the tank.

  • Minimum Tank Size: 75 gal (284 L)
  • Suitable for Nano Tank: No
  • Substrate Type: Sand/Gravel Mix – These eels like to dig so make sure substrate is a sand or fine gravel that won’t cause injury.
  • Lighting Needs: Low – subdued lighting – In moderately lit tanks, provide floating plants to help subdue the light.
  • Temperature: 74.0 to 82.0° F (23.3 to 27.8&deg C)
  • Range ph: 6.0-8.0
  • Hardness Range: 6 – 25 dGH
  • Brackish: No
  • Water Movement: Moderate
  • Water Region: Bottom – Black Spotted Eels will spend most of their time on the bottom of the aquarium.

Social Behaviors

They are a nocturnal species but are generally peaceful and shy. This fish is not very aggressive but due to their large size mouths, they may eat smaller tank mates. Other members of the species and family will generally not be tolerated, and Black Spotted Eels are usually best kept singly unless breeding is intended. In a large enough tank with many areas to hide multiples can be kept.

  • Venomous: No
  • Temperament: Peaceful – This fish is not very aggressive but smaller tankmates may be eaten.
  • Compatible with:
    • Same species – conspecifics: Sometimes – Only keep more than one if you intend to breed, or if the tank is very large with enough room for each eel to have undisturbed territories.
    • Peaceful fish (): Monitor – Tank mates need to be large enough to not be eaten.
    • Semi-Aggressive (): Monitor
    • Aggressive (): Monitor
    • Large Semi-Aggressive (): Monitor
    • Large Aggressive, Predatory (): Threat
    • Monitor
    • Shrimps, Crabs, Snails: May be aggressive
    • Plants: Monitor – These eels will uproot most planted plants.

Sex: Sexual differences

Sexual differences are unknown and it is almost impossible to identify the sexes, though a mature female may be more full bodied.

Breeding / Reproduction

The Black Spotted Eel has not been bred in captivity. Only a few spiny eels have been bred in the aquarium, possibly because they are generally kept singly rather than in a group where a male and female can find each other. Though it is not documented what makes them spawn, trying to emulate the bounty of the flood season can help stimulate breeding behavior. Feed more and higher quality food than you normally would and providing an influx of clean water. Their courtship lasts for several hours, where they chase each other and swim in circles.

The eggs are deposited among floating plants. They are sticky so will adhere to the plants and then hatch in 3 to 4 days. The fry becoming free swimming a few more days after that and should be fed nauplii. The fry are something of a challenge to raise as they are susceptible to fungal infections. Regular water changes and the use of an antifungal water treatment can help.

  • Ease of Breeding: Unknown – This fish has not been bred in the home aquarium, however it is presumed that their breeding process is similar to other Spiny Eels.

Fish Diseases

Eels are prone to diseases caused by parasites and fungus, so take caution when introducing these fish to an established tank. Eels are also very sensitive to medications used to treat many diseases; a separate hospital tank is needed. Very low water temperatures and condition changes can also cause stress to this fish which makes them even more prone to disease. Take great care when netting eels as they have very delicate and scraps can make them even more prone to disease.

The most common disease that an eel is susceptible to is Ich. Ich is short for Ichthyophthirius, also known as “white spot disease”. It is a parasite that can attack nearly all aquarium fishes, but you’ll find that Eels are often the first to be attacked. Take great care in treating ick as eels are very sensitive to the medications used to treat it. Often the dose is half of what is normally used. If nervous or unsure about medications, use Reef safe medications.

An outbreak of disease can often be limited to just one or a few fishes if you deal with it at an early stage. When keeping these sensitive types of fish, it is common to catch deteriorating water conditions and disease before other fish are affected. The best way to proactively prevent disease is to give your Black Spotted Eel 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 will is more likely to acquire disease.

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. 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.


Black Spotted Eels, also called the Polka Dot Eel or Spotted Spiny Eel, are commonly available.



Featured Image Credit: Kai.Kasinee, Shutterstock