The truth about spiny eels; 

With the exception of the Kuhli loaches, spiny eels are easily the most popular and widely sold eel-like fishes in the hobby. To their credit, they are relatively hardy and often attractively coloured creatures, but even so many aquarists' attempts at keeping these fish have met with failure. Like many other 'oddball' fishes, part of the problem is lack of information, with most aquarium books either saying little about them or nothing at all. In this article, we'll look at the most commonly traded species and review their requirements in the home aquarium. 

Spiny eels belong to a family of their own, the Mastacembelidae, not particularly closely related to anything else. Only three genera are currently recognised: Macrognathus, Mastacembelus, and Sinobdella. Older aquarium books sometimes mention three additional genera, Aethiomastacembelus, Afromastacembelus, and Caecomastacembelus, but these are now considered to be junior synonyms of Mastacembelus.

Two different genera of spiny eels are exported from Asia, Macrognathus and Mastacembelus. Distinguishing between the two genera is not easy, and identifying the different species is even more difficult. As a result, the aquarium literature is hopelessly confused, with fishes routinely misnamed. This makes it difficult for aquarists to determine which species they have. Given that the traded species range in size from 15-100 cm, and vary in behaviour from the meek and shy through to aggressive and predatory, identifying a fish before purchasing is absolutely essential. 

The one species that can't be misidentified is Mastacembelus erythrotaenia, a stunning animal known as the fire eel. This fish is charcoal grey in colour marked with red and yellow stripes and spots. 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. Mastacembelus erythrotaenia is the largest of all the spiny eels, getting to around 100 cm in the wild. An aquarium specimen might not even get to about two-thirds that size in captivity, but it will still need a tank with a volume of at least 300 litres to do well. 

Moving on from the fire eel is where things get complicated. The tyre-track eel is usually identified as Mastacembelus armatus in aquarium books, but in fact a different species, Mastacembelus favus, actually seems to be the one that gets imported most often. Both are light brown with dark brown markings, but on Mastacembelus armatus the markings are concentrated on the top half of the body, leaving most of the belly bare, whereas Mastacembelus favus is marked right down to the belly. Otherwise, the biggest difference between the two species is size, with Mastacembelus favus reaching up to 70 cm in length, but Mastacembelus armatus up to 90 cm. 

Within the genus Macrognathus, three species are regularly kept as aquarium fish. Of these, Macrognathus circumcinctus is probably the most easily identified. It is basically light brown covered with creamy speckles, but with a series of roughly vertical bands along the flanks. As its common name, the half-banded spiny eel, suggests, these bands are incomplete, and only appear about halfway down the side of the flank. From there, they taper towards the belly where they join with the corresponding band from the other side of the fish. The dorsal fin is light brown, and though it has around ten faint eyespots where it joins the body, these are much less prominent that those of the other commonly traded Macrognathus. 

Macrognathus aral is sometimes called the one-stripe spiny eel, though this name is somewhat misleading. Its basic colour is olive to light brown, and while there is certainly a single band running horizontally along each flank, there is also a dark band along the dorsal surface. These bands are rather irregularly coloured, often being speckled or being darker on the edges and lighter in the centre. The dorsal fin bears a few (typically four) eyespots that are light brown around the edge and dark brown in the middle. 

Macrognathus siamensis is known as the peacock spiny eel. In some aquarium books this fish is referred to as Macrognathus aculeatus and Macrognathus pancalus, but these are quite different species that are rarely, if ever, traded as aquarium fish. Macrognathus siamensis is light brown in colour with a thin, cream-coloured stripe running from the eye to the base of the tail. The dorsal fin is embellished with about six eyespots, but otherwise this fish is much less dramatically coloured than any of the other Asian spiny eels. Nonetheless, this species has proven to be hardy and adapts readily to aquarium life, and at no more than 30 cm in length, it makes a good choice for someone trying out spiny eels for the first time.

Africa is well endowed with spiny eel species, but surprisingly few are ever traded. The only African spiny eels you are likely to find in the shops are the ones from Lake Tanganyika, in particular Mastacembelus moorii and Mastacembelus plagiostomus. Mastacembelus moorii is sometimes known as the mottled spiny eel though sometimes is simply labeled as the Tanganyikan spiny eel. It is a beautiful fish, with a dark snakeskin pattern set against a creamy-brown background colour. The pectoral fins are yellow. Mastacembelus plagiostomus is also from Lake Tanganyika and is another very attractive fish, this time with a cream to salmon coloured body and dark brown, saddle-like bands running over the back and about two-thirds of the way down the flanks. This handsome fish doesn't have a common name as such, but if you see it, it'll probably be called either the Tanganyikan spiny eel or simply the African spiny eel.

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 the use of salt to prevent skin infections that got the idea that these were brackish water fish stuck in the hobby. In reality, spiny eels are normally found in freshwater rivers and streams, and only a very few additionally occur in brackish waters. Even then, they are tolerant of only slightly saline water. For the Asian species, soft to moderately hard, acidic to slightly alkaline water will suit them best. The riverine African species need similar conditions, while the species from Lakes Malawi and Tanganyika will need hard, alkaline water to do well. 

Smaller spiny eels, particularly those of the genus Macrognathus, are inveterate burrowers, and must be kept in tanks with a sandy substrate. Failure to do so causes a number of problems, the most serious of which is that the fish can scratch its skin when trying to dig into the gravel, and in doing so expose itself to opportunistic bacterial infections. These infections are difficult to cure, and invariable end up with the death of the fish. The larger species, like the fire eel and tyre-track eel, should be kept in a sandy aquarium when small, but the adults do not dig much and will be equally happy if well provided for with alternative hiding places, such as caves and crevices between large rocks. All spiny eels appreciate floating plants, and the smaller species in particular will 'burrow' into tangles of things like hornwort where they can be more easily observed than when they hide in the sand. Indeed, because spiny eels are such enthusiastic burrowers, there's probably no good reason to bother with rooted plants. Instead, add your plants from the top downwards, using floating plants and things like Java ferns and Anubias attached to smooth pieces of bogwood. 

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. The Asian species are generalists, and take a wide variety of foods, from bloodworms to small fish, but earthworms are a particular favourite. The African species tend to be more piscivorous, though chunky live foods such as earthworms and river shrimps can be used initially, and eventually they should take frozen prawns and mussels. 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. 

Beyond starvation and skin damage, there are two 'sudden death' risks to be aware of when keeping spiny eels. 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. Alternatives are available formulated in tanks containing catfish, Mormyrids, and stingrays, and these should be used instead. If in doubt, ask your retailer for medications suitable for use in reef tanks.

Spiny eels are generally shy and ignore tankmates too big to eat. Towards their own kind, spiny eels run from being hostile at one extreme to relatively peaceful at the other. As a broad rule, species of Mastacembelus tend to be territorial, whereas species of Macrognathus are much more sociable. Nonetheless, while Macrognathus usually get along, in small groups (twos or threes) there's still the potential of bullying, especially if the tank is small and lacks hiding places. When kept in groups, though, spiny eels tend to be more outgoing and more likely to settle down quickly.  

Another benefit of keeping spiny eels in groups is the opportunity for breeding. 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. Perhaps the abundance of food and the influx of clean water mimic the 'rainy season' of their natural habitat? Certainly species like Macrognathus aral are known to spawn only during the monsoon. 

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.

A sandy substrate is very important when keeping spiny eels. Aquarists have a variety of options to choose from, but perhaps the best is to use silica sand (sometimes called silver sand). A smooth variety of this stuff can be bought very inexpensively from most garden centres, where it is normally used as a soil additive for pot plants. You only need enough for the eel to dig into, in which case around 5 cm will be adequate for a spiny eel 15-20 cm 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.

Mastacembelus erythrotaenia 

Common name:          Fire eel

Origin:                        South East Asia

Maximum Size:          100 cm

Water requirements: Freshwater, ideally soft to moderately hard, pH 6.0 -- 7.5

Food:                          Various invertebrates and small fish

Social behaviour:        Very territorial and cannot be kept with other spiny eels; works well with large but peaceful fish such as clown loaches, tinfoil barbs, and Plecs

Breeding:                   Not yet bred in aquaria

Mastacembelus favus 

Common name:          Tyre-track eel

Origin:                        South East Asia

Maximum Size:          70 cm

Water requirements: As Mastacembelus erythrotaenia

Food:                          As Mastacembelus erythrotaenia

Social behaviour:        As Mastacembelus erythrotaenia

Breeding:                   Not yet bred in aquaria

Macrognathus circumcinctus 

Common name:          Half-banded spiny eel

Origin:                        South East Asia

Maximum Size:          15 cm

Water requirements: Soft, acid water preferred

Food:                          Small worms and insect larvae

Social behaviour:        Peaceful, even sociable, and unlikely to eat any tankmates larger than a female guppy

Breeding:                   Not yet bred in aquaria

Macrognathus aral 

Common name:          One-stripe spiny eel

Origin:                        South Asia, from Pakistan to Burma

Maximum Size:          Supposedly up to 60 cm, but normally much smaller

Water requirements: Freshwater preferred, but slightly brackish water tolerated

Food:                          Various small animals including fish, but especially bloodworms and other insect larvae

Social behaviour:        Fairly peaceful and tolerant, can be kept in groups provided the fish are not overcrowded

Breeding:                   Has been bred under lab conditions, but not yet in home aquaria

Macrognathus siamensis 

Common name:          Peacock spiny eel

Origin:                        South East Asia

Maximum Size:          Up to 30 cm

Water requirements: Soft, acid water preferred

Food:                          As Macrognathus aral

Social behaviour:        Fairly peaceful and tolerant, can be kept in groups provided the fish are not overcrowded

Breeding:                   Occasionally bred in aquaria

Mastacembelus moorii 

Common name:          Mottled spiny eel

Origin:                        Lake Tanganyika

Maximum Size:          40 cm

Water requirements: Slightly hard and alkaline (pH 7.5)

Food:                          Prefers small fish, but will eat earthworms, frozen prawns, and other chunky invertebrates

Social behaviour:        Territorial towards other spiny eels, otherwise ignores fish too big to eat

Breeding:                   Not yet bred in aquaria

Mastacembelus plagiostomus 

Common name:          Tanganyikan spiny eel

Origin:                        Lake Tanganyika

Maximum Size:          30 cm

Water requirements: As Macrognathus moorii

Food:                          As Macrognathus moorii

Social behaviour:        Fairly peaceful and tolerant, can be kept in groups provided the fish As Macrognathus moorii

Breeding:                   Not yet bred in aquaria