Fire Eel

The fire eel (Mastacembelus erythrotaenia) is a large freshwater fish found in Cambodia, Indonesia, Laos, Malaysia, Thailand, and Vietnam.

The Fire Eel  is one of the largest spiny eels. It has a brown/black elongated body with a pointed snout and is marked with bright orange to red horizontal broken lines and spots. Its dorsal and anal fins have orange to red margins.

Common Name Fire Eel, Spiney Eel
Scientific Name Mastacembelus erythrotaenia
Origin Southeast Asia
Max Size (in inches) 40
Community Safe Yes, with caution
pH Range 6.0-8.0
Diet Omnivore
Min Tank Size (in gallons) 100
Temperature Range 74-82
Difficulty Moderate
Fire Eel - Mastacembelus erythrotaenia - Medium

Fire Eel – Mastacembelus erythrotaenia – Medium

One of the most persistent myths in the fishkeeping hobby is that spiny eels need brackish water; they do not. The origins of this myth are unclear, but it is possibly from the use of salt to prevent skin infections that people got the idea that these were brackish water fish. In reality, spiny eels are normally found in freshwater rivers and streams in the wild.

Freshwater eels are generally bottom dwellers that will often bury in the substrate or hide in crevices and caves. Provide small grain; smooth gravel or sand so that the fish can burrow safely; as well as tubes or rock crevices for hiding places.

These fish are peaceful when kept with large tankmates; but will often consume small fish. These eels feed primarily on insect larvae; worms; and occasionally; plant material in their natural habitat. In the aquarium; they can be fed most sinking pellets and other meaty foods.

When they grow larger; they will eat small fish; so be sure that any fish kept with the eel are too large to swallow.

Freshwater eels do have an escape risk; keep a tightly-fitting lid on the tank to keep the eels in the aquarium. Most species are nocturnal and will hide for most of the day; becoming more active at night.

The fire eel is a moderately advanced species to keep. It belongs to the spiny eel family, and is extra sensitive to medicines in the aquarium. Due to the large adult size of this fish, larger aquariums are mandatory.  Probably a 55 gallon tank is the bare minimum, but 100 gallon or larger is preferred due to their large mature size.

Fire Eels are very sensitive to the substrate in tanks, and can be easily scratched by gravel or sharp rocks. A sand substrate is preferred, as this is a burrowing fish. Although it is possible to keep one in a gravel-bottom tank, it will get scratched constantly and might get seriously hurt due to bacterial infections or other skin openings.

The best option  is to use silica sand (sometimes called silver sand). A smooth variety of this can be bought very inexpensively from most garden centers, where it is normally used as a soil additive for potted plants.

You only need enough for the eel to dig into, in which case around two inches will be adequate for a spiny eel 6-8 inches in length.

As these fish like to dig and will quickly find any worms or shrimps that get lost in the sand, there’s no great risk of the sand becoming anaerobic, but adding a few Malayan livebearing snails will keep the sand spotlessly clean between water changes.

Every few weeks, give the sand a stir, and siphon up any mess you find.

Keep in mind that any and all plants might be uprooted when it is burrowing, so it is very difficult to keep this fish in a planted tank. Try to provide your fire eel with plenty of hiding places so it has a house and a place to go if it becomes stressed or scared.

Fire Eels are well known for hopping the border. Be sure that you have your whole tank covered, including the filter(s) intake(s)/output(s) if possible. If you have a bigger specimen, you will need to compensate for its strength by re-enforcing the top. Many valuable eels are lost due to un-educated owners.

This fish does not do very well with other spiny eels, especially as adults. Although it has been tried, very few have had success keeping Fire Eels with other eels and conspecifics.

A fire eel will do well in a community tank with fish about the same size or slightly larger/smaller than it is. It is generally not a good idea to mix fire eels with larger aggressive cichlids due to aggression issues and battles for food.

Although usually a shy fish at first, it is not uncommon to find to find your fire eel swimming around, especially at night. Your fire eel may be more active if kept at the upper ranges of water temperatures above.

The fire eel is an eel-like fish, but is not truly an eel. They are usually brown, and have either red dots or red lines running on both sides of their body when mature, and can be anywhere from ¼ an inch to several inches thick. Yellow fire eels also occur, although it is more of a faded red.

Typically, there are yellow stripes on the face, becoming red a short way behind the head. On some fish the red stripes are continuous and run all the way to the tail, but on others the stripes are broken into shorter sections or turn into large round spots along the flanks. The dorsal and anal fins are edged with red.

Immature Fire Eels are called elvers, and are not as brightly colored, being brown where the stripes become red as adults. They only reach their true coloring when they reach maturity.

The fire eel is a very unique fish. It is a mainly nocturnal fish and usually only comes out at night to feed. When first introduced into an aquarium, a fire eel may not eat for several days or usually weeks. This is not uncommon.

Spiny eels have a well-earned reputation for being difficult to feed. As a rule, they are shy upon import and it can take weeks, even months, before they truly settle down.

It is therefore important that the aquarist provides them with the right kinds of food at the times when the spiny eel is likely to be foraging. Since spiny eels are primarily nocturnal, during the settling-in period they should be fed at night, with the aquarium lights out.

Since spiny eels are so shy, it isn’t a good idea to combine them with things like catfish or loaches that would take the food more quickly, at least not until your spiny eel is settled in and feeding freely. 

They feed on worms; insect larvae; crustaceans; fish; and sometimes plant matter. Earthworms are a favorite.

It is best to try and get it to feed at night when the tank lights are off at first, and then slowly adjust it to your feeding schedule. In addition to a large, well-covered tank with a soft substrate, one must not overfeed this fish; it could very well pass away due to gluttony.

Breeding of Fire Eels has been achieved before, but due to large adult size, a tank size of several hundred gallons is needed, and it is usually only achieved by advanced breeders or extremely dedicated hobbyists.

Only a few spiny eels have spawned in captivity at all, though this is likely more about them being rarely kept in groups than in any intrinsic difficulty.

Identifying the two sexes is the first challenge, and this is impossible with immature fish. One fully grown, females are obviously more deeper-bodied than the males.

The exact spawning trigger is unknown, but feeding the fish well and performing substantial water changes appear to be important. In the wild, they only spawn during the monsoon season, and frequent water changes is thought to emulate the rain.

Perhaps the abundance of food and the influx of clean water mimic the ‘rainy season’ of their natural habitat?

Courtship is a lengthy, elaborate process that lasts several hours. The fish chase one another and swim around in tight circles before spawning.

The sticky eggs are deposited among the leaves or roots of floating plants such as water hyacinth. Up to a thousand eggs are produced, about 1.25 mm in diameter, and these hatch after three or four days.

The fry become free swimming another three to four days later, at which point they need tiny foods such as radiolarians, Cyclops nauplii, and hard-boiled egg yolk.

A particular problem with newly hatched spiny eels is a certain susceptibility to opportunistic fungal infections. Regular water changes are very important, and the use of a safe antifungal agent like Pimafix might also be worthwhile.

The Fire Eel is a great fish that would make an excellent addition to certain tanks, but it is not a fish for the beginner.

Beyond starvation and skin damage, there are two ‘sudden death’ risks to be aware of when keeping Fire Eels, or any spiney eel.

The first is escape: these fish are adept at both jumping and wriggling through small cracks. Uncovered aquaria will simply be left behind as the fish makes a suicidal leap for the floor, so such tanks are just not an option; but even in a covered aquarium, any tiny space between the hood and the tank will be exploited.

This is especially important to consider where external filters are used, as the gaps through which the pipes pass can be used by spiny eels bent on escape.

The other critical danger is in the use of medications. Spiny eels can respond badly to copper-based medications, often used for things like whitespot.

If provided with a large enough tank and proper covering and tankmates, this fish will grow to be a large, breathtaking specimen.

It is a fish with a very unique personality, and many owners feed their fire eels by hand. A true oddball fish, it does require certain tank specifics and is very hardy once rooted in a healthy, well kept aquarium.

Further Reading:

Eels: An Exploration, from New Zealand to the Sargasso, of the World’s Most Mysterious Fish

Giant Morays and Other Extraordinary Eels (Creatures of the Deep)

Keeping Moray Eels in Aquariums

500 Freshwater Aquarium Fish: A Visual Reference to the Most Popular Species

Penn Plax Action Air Diver & Eel, Aquarium Decoration