Unicolor Spiny Eel (Mastacembelus unicolor)

Meet the plain-coloured giant among spiny eels

The Unicolor Spiny Eel (Mastacembelus unicolor) is a large, long-bodied spiny eel from Indonesia and neighbouring parts of South-East Asia. It belongs to the family Mastacembelidae, the same family as the Fire Eel and the Tyre-Track Eel, and shares their pointed, fleshy snout, the row of small separate spines along the back and the long soft dorsal and anal fins that run into the tail. Where the Fire Eel is striped with red, this fish is an even brown to dark olive, which is where the name unicolor, "one colour", comes from.

It is a true oddball for a large, quiet aquarium: nocturnal, partly buried in sand by day and increasingly confident at feeding time once it settles. Despite the name, spiny eels are not true eels but a separate group of freshwater fish. FishBase lists the species as harmless to people, and Seriously Fish describes the dorsal spines of its close relatives as harmless to humans too.

Recognise the species

Aquarium Glaser, which has imported the species several times, describes it as looking very much like a Fire Eel without the red, and notes that its first unicolor were sent by mistake as M. erythrotaenia. Ralf Britz of the Natural History Museum in London identified those fish as most probably M. unicolor, "or what is currently thought to be that species", so the scientific picture may still change. The species was first collected in western Java by the naturalists Kuhl and van Hasselt in the early 1820s and later described by Cuvier from their material.

Browse our Mastacembelus spiny eels to compare it with the Fire Eel, the Zig-zag Spiny Eel and the Zebra Spiny Eel, or see all our spiny eels and other loaches and oddball fish.

Know what one purchase brings

One purchase supplies one live Unicolor Spiny Eel. The listing has a single option rather than graded sizes, and this species reaches the trade at very different sizes: Aquarium Glaser reports juveniles of 10–20 cm, a younger fish of 20–25 cm and adults of 50–60 cm. If the size at dispatch matters for your setup, ask us before ordering and we will tell you what is currently available.

The photographs are illustrations prepared from a reference image of the species. They show the body form and plain colouring, not the individual you will receive. Young fish can be noticeably redder or darker than adults, and the pale fin edge may be missing in fish below about 20 cm.

Check four things before you order

1. Space: an aquarium with a base of at least 120 × 60 cm is ready, or firmly planned for the adult.
2. Lid: every gap around pipes, cables and filters can be closed.
3. Floor: a soft sand bed deep enough for the eel to bury its body.
4. Food: you can supply frozen meaty foods such as krill, mussel and prawn every week.

Read our delivery and Live Arrival Guarantee information before you buy, and make sure the aquarium is fully matured before the eel arrives.

Plan the aquarium for a 55–60 cm adult

The sources agree that this is a big fish. FishBase records a maximum of 55 cm total length, and Aquarium Glaser describes the species as reaching around 60 cm. Plan the aquarium for a fish of 55–60 cm, not for the size it arrives at.

No species-specific aquarium profile exists for M. unicolor, so the tank size here comes from its closest well-documented relatives. Seriously Fish gives the same minimum for the Fire Eel and the Tyre-Track Eel: a base of 120 × 60 cm, about 432 litres, and notes that juveniles can start in a smaller tank. Both of those species can grow larger than the unicolor, so that footprint gives a 55–60 cm eel room to stretch, turn and patrol.

Measure the footprint, not just the litres

Use our tank volume calculator to check the real capacity of your aquarium, remembering that sand, rock and wood displace water. A juvenile can be grown on in a smaller, well-filtered tank, but only if the larger aquarium is a firm plan rather than a hope.

OATA's guidance on very large fish applies here: plan for the adult size from the start, because many big fish grow quickly, and only take one on if you can look after it for its whole life. Never release an aquarium fish into the wild; OATA points out that this is illegal and usually ends badly for the animal.

Build a soft, dark floor with plenty of refuges

Spiny eels spend the day buried or hidden. Seriously Fish recommends sand for large Mastacembelus because they often bury themselves, and the Maidenhead Aquatics fish database describes the Fire Eel spending the day partly buried with only its snout showing. Practical Fishkeeping accepts smooth gravel as an alternative for the Tyre-Track Eel, but fine, rounded sand is what lets a spiny eel burrow properly.

Lay out the tank by function

Seriously Fish lists bogwood, rock caves and PVC piping as hiding places and recommends dimmer lighting to encourage the fish to emerge. Maidenhead Aquatics adds two practical points: set heavy decor on the glass base before you add the sand, because large spiny eels can undermine it, and fit a timed blue moonlight that comes on just before the main lights go off, so you can watch the eel's night-time activity.

Because large spiny eels dig, plants tied to wood and floating plants tend to last better than plants rooted in the sand. Leave the front of the tank open so you can see the eel feed.

Close every gap

Seriously Fish says a close-fitting hood is essential because spiny eels can find their way through the smallest of gaps. Maidenhead advises tight cover slides with no openings where equipment enters the tank, and suggests plugging holes with oversized pieces of filter sponge. Practical Fishkeeping makes the same point: spiny eels often try to escape. Check the lid again after every maintenance session.

Keep warm, soft to medium water stable

FishBase records the species as a tropical freshwater fish, mainly from upland streams and also found in lakes during the dry season, but it gives no water values. The ranges below come from the Seriously Fish profiles of the Fire Eel and Tyre-Track Eel and from Maidenhead Aquatics. They overlap well, so a stable mid-range setting suits this fish.

Set your targets

UK tap water ranges from soft to very hard depending on where you live, so check your water company's report for your postcode, then test the aquarium itself. If your water is on the hard side, Maidenhead's upper limit of 18 dH and Practical Fishkeeping's note that the Tyre-Track Eel adapts to harder water show that the genus is adaptable; a steady reading matters more than a perfect one. Our water matcher compares your readings with the range, and the water chemistry guide explains each value.

Filter for a large meat-eater and change water weekly

A 55–60 cm eel eating prawn, mussel and krill produces a lot of waste. For the Fire Eel, Seriously Fish suggests filtration that turns the tank volume over four to five times an hour, with fairly gentle flow because the species lives mainly in still water. For a 432-litre aquarium that is roughly 1,730–2,160 litres per hour, which Seriously Fish meets with two external filters rather than one.

Follow a weekly routine

1. Test ammonia, nitrite and nitrate before you start.
2. Change at least 25% of the water, as OATA advises for large fish; Maidenhead recommends frequent partial changes for big spiny eels.
3. Treat new water with a conditioner that removes chlorine and chloramine, and match its temperature.
4. Siphon leftover food from the sand surface, especially near the caves where the eel eats.
5. Rinse filter media only in removed aquarium water, never under the tap.
6. Check the lid, heater and every cable entry before you close up.

Protect the heater with a guard, or run it in a sump or external unit, so a resting eel cannot lie against it; OATA advises protecting heaters in large-fish tanks. Our guide to aquarium filtration covers sizing filters for big fish, and the nitrogen cycle guide explains why the tank must be mature before the eel arrives.

Feed meaty foods at dusk

Seriously Fish describes wild Fire Eels as omnivores that eat smaller fish, aquatic invertebrates, some plant matter and detritus. In the aquarium, large Mastacembelus do best on meaty foods: Seriously Fish lists prawn, krill and lancefish, and warns that small live and frozen foods alone are unlikely to sustain larger fish. Practical Fishkeeping adds that bigger spiny eels appreciate earthworms and that some take sinking tablet foods. Maidenhead notes that dried foods are rarely accepted, so do not rely on flake or pellets.

Build a measured meal

Feed once a day, as OATA recommends for large fish, in the evening just after the main lights go off. At first, place the food close to where the eel hides. Offer what it finishes within a few minutes and siphon out anything left the next morning, because uneaten meaty food quickly fouls the water.

Our frozen mussel meat, large frozen krill and live earthworms cover the main foods. Thaw frozen food in a little tank water first.

Introduce it to a dark, mature aquarium

This species reaches the trade as imports from Indonesia. OATA notes that wild-caught fish can be less tolerant of water outside their normal range, may need live or fresh food before they reliably take prepared diets, and should be acclimatised with care and patience.

Use OATA's floating-bag method

1. Switch off the aquarium lights and keep the bag out of bright light.
2. Float the sealed bag for 10 minutes so the temperatures match.
3. Add small amounts of aquarium water to the bag over up to 20 minutes.
4. Let the eel swim out into the tank, adding as little bag water as possible.
5. Leave the lights off for the rest of the day and check the lid is closed.

Our step-by-step acclimatisation guide covers the method in more detail, including the slower drip method for sensitive fish.

Use a first-week checkpoint plan

Do not worry if you barely see it at first; a new spiny eel can hide for days. What matters is that food disappears overnight and the body stays full. If it ignores frozen food after several days, try a live earthworm or live bloodworm, as OATA suggests for wild-caught fish.

Watch its body and skin and act early

OATA lists warning signs that apply to every fish: hanging at the surface or lying abnormally, colour changes, faster or slower gill movement, white spots or fluffy growths, damaged fins, weight loss and reduced feeding. In a spiny eel, pay particular attention to the skin, which is in constant contact with the sand, and to the belly, which shows whether it is eating.

Pair each sign with an action

Before using any treatment, read the label for warnings about eels or sensitive species and correct the water first. When you contact us, share the tank size, temperature, pH, ammonia, nitrite and nitrate readings and what you have seen.

Keep one per tank with fish too big to eat

Seriously Fish calls its large relatives generally peaceful but warns that tank mates must be too large to be prey, and recommends one per tank because they may be aggressive to their own kind; groups of five or more sometimes spread the aggression, but only in a very large aquarium with many refuges. Maidenhead adds that adult Fire Eels will eat small fish overnight. The same rules apply to a 55–60 cm unicolor.

Choose companions with care

Maidenhead's suggested companions for the Fire Eel are large gouramis, larger peaceful botiid loaches, knifefish, Geophagus and severums. From our range, the Severum, the Surinam Geophagus and a group of Clown Loaches fit that list, provided the tank is large enough for every adult. Check a plan with our tank mates tool and stocking calculator before you buy, and see more oddball spiny eels if you want to compare species.

Expect no visible sex differences

Spiny eels are hard to sex. Maidenhead says mature female Fire Eels in breeding condition may look plumper, and that breeding in home aquariums is rare; Seriously Fish describes the few Fire Eel spawnings as accidental. There are no published aquarium breeding reports for M. unicolor, so buy it as a single display fish rather than as a breeding project.

Answer Unicolor Spiny Eel buying questions

What is a Unicolor Spiny Eel?

It is Mastacembelus unicolor, a large freshwater spiny eel (family Mastacembelidae) from Indonesia and neighbouring parts of South-East Asia. It looks much like a Fire Eel without the red: an even brown to olive body, a long pointed snout and a pale edge to the fins in larger fish.

How big does it grow?

Plan for 55–60 cm. FishBase records a maximum of 55 cm total length and Aquarium Glaser describes the species as reaching around 60 cm.

What size aquarium does it need?

A base of at least 120 × 60 cm, about 432 litres, which is what Seriously Fish gives for its close relatives the Fire Eel and Tyre-Track Eel. A juvenile can be grown on in a smaller, mature tank while the large aquarium is prepared.

Is it a real eel, and is it dangerous?

Spiny eels are not true eels; they are a separate family of freshwater fish. FishBase lists the species as harmless, and Seriously Fish describes the dorsal spines of its relatives as harmless to humans. Move it with a soft net or a container, as for any large fish.

What does it eat?

Meaty foods: frozen krill, prawn, mussel, mysis and bloodworm, with lancefish or smelt and earthworms for larger fish. Dried foods are rarely accepted. Feed once a day, in the evening, and remove leftovers.

Can I keep two together?

Usually not. Seriously Fish advises one per tank because large Mastacembelus may be aggressive to their own kind; groups of five or more sometimes work, but only in a very large aquarium with many refuges.

Which tank mates are safe?

Peaceful fish too big to swallow, such as severums, Geophagus eartheaters and larger loaches in a tank big enough for all of them. Avoid small fish, shrimps and anything that can fit in its mouth, because large spiny eels hunt at night.

How big is the eel I will receive?

One purchase supplies one live eel. The listing has one option rather than graded sizes, and imports of this species range from juveniles to adults, so ask us before ordering if you need to know the current size.

Consult the care references

• FishBase: Mastacembelus unicolor — accepted name, family, distribution, habitat, maximum recorded length and conservation status.
• Aquarium Glaser: Mastacembelus unicolor (2018 import) — the first imports, how the fish differs from the Fire Eel and the expert identification.
• Aquarium Glaser: Mastacembelus unicolor (2025 import) — adult size, juvenile colour phases, the pale fin edge and the history of the species.
• Seriously Fish: Mastacembelus erythrotaenia — tank size, water range, sand, cover, lid, filtration, diet and compatibility of the closest relative.
• Seriously Fish: Mastacembelus armatus — tank size, water range, layout, diet and one-per-tank advice for another large Mastacembelus.
• Maidenhead Aquatics fish database (fishkeeper.co.uk): Fire Eel — water range, foods, tank mates, decor placement, moonlight and lid advice for large spiny eels.
• Practical Fishkeeping: Mastacembelus armatus — substrate, escape-proofing, earthworms and harder-water tolerance in large spiny eels.
• OATA: How to look after very large fish — UK guidance on very large fish: water quality limits, maintenance, feeding, acclimatisation, health signs and lifelong commitment.
• OATA: How to look after wild caught freshwater fish — UK guidance on acclimatising, feeding and observing wild-caught freshwater fish.
• Tropical Fish Co delivery policy — current dispatch days and Live Arrival Guarantee terms.