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Freshwater Moray Eels
Although traded as freshwater fishes, these fishes really need moderately brackish water to do well, and most probably prefer salinities approach normal seawater. As such, they can mix well with scats, monos, and other large, high-salinity brackish water fish. However their social behaviour can be unpredictable. Some aquarists have kept groups without any problems, while others have found them to be much more territorial and aggressive. Similarly some specimens ignore large fish too big to swallow whole, while others repeatedly attack their tankmates.
Thomas Flörkemeier reports that Gymnothorax tile is killled by copper-based medications. Aquarists keeping freshwater morays should consider this when using many of the commercial medications and act accordingly.
Hunger Strikes & Feeding
Moray eels almost invariably stop feeding when kept too long in water with insufficient salinity. But provided the fish is being maintained at a middling salinity (around SG 1.010) these fish tend to be reliable, even aggressive, feeders. There is no need to offer them live feeder fish; river shrimps can be used to help settle in a new specimen, and after that it should take any sort of chunky seafood. Mixed frozen seafood (typically squid, prawn and mussel) sold for human consumption will make an ideal staple diet.
Bear in mind that morays are largely nocturnal, so the evening is likely to be the best time to feed them. Morays are said to become confused when too much food is put in the tank at once, perhaps because the source of the scent is difficult to locate, so offer only small amounts at a time.
Moray eels have sharp teeth and mildly toxic saliva; while not aggressive, they should be treated with respect.
Common Freshwater Moray
Gymnothorax tile is by far the most commonly sold ‘freshwater moray’ in the hobby. It is pinkish-grey in colour and covered in many tiny cream spots. The biology of this species is not fully understood but it is believed to be anadromous, meaning that it inhabits brackish and marine waters as an adult but breeds in brackish or fresh water. It is a large fish, potentially reaching 60 cm (24 inches) in length.
Other Freshwater Morays
A related species, Gymnothorax polyuranodon, is sometimes sold and is distinguished by a giraffe-like pattern of dark brown patches on a cream coloured body. It is believed to be a catadromous species, that is, while adults may inhabit brackish or fresh waters for extended periods, they return to the sea to breed. Aquarists have found that this species does best in at least moderately brackish water (SG above 1.010); if the salinity is too low this fish will stop eating and becomes lethargic. It is by far the biggest of the ‘freshwater morays’, the largest specimens exceeding lengths of 90 cm (about 3 feet).
Echidna rhodochilus is gets to about 30 cm (12 inches) in length and is basically a light brown in colour, darkening on the back; the most characteristic feature are light patches on the sides of the mouth. Like all moray eels, these fish are predators, but they are especially fond of small crustaceans such as shrimps (river shrimp are ideal). Species of Echidna have flattened, shell-crushing teeth rather different to the needle-like teeth of most other morays.