Ask the WWM Crew
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Their fierce look, their snakelike elongated body and their general oddball attitude lure many fish keepers into buying them. But freshwater moray eels are not really what their common name pretends they are. Put into the standard community tanks they sometimes start to reduce the stocking, but in most cases simply starve or die due to various diseases. That's pretty sad, because under the right conditions and with appropriate care they can be wonderful fish living for more than ten years.
The by far most common moray eel sold as a freshwater fish is Gymnothorax tile, it is often simply referred to as 'freshwater moray eel'. Sometimes it is labeled 'Indian mud moray', 'snowflake eel' (not to be confused with the other 'snowflake eel' Echidna nebulosa) or 'gold dust moray'. It is grey and has numerous yellow to golden spots spread on the dorsal and lateral part of the body. With age the yellow spots become smaller giving the adults a more or less uniform grey appearance. They are common in the Sundaban mangrove swamps in East India, but they are also distributed in Indonesia, the Philippines and the Andaman Islands. The species reaches a maximum length of 60 cm (24 inches).
Rarely sold are two possible variations of Gymnothorax tile. They may also represent different species. The variation 'albino gold' has a very bright, whitish background colour, possibly a xanthoristic form. The other possible variation has a red stripe at the lower jaw and is traded as 'red stripe'.
A species often confused with the Indian mud moray is Gymnothorax polyuranodon. In contrast to Gymnothorax tile, it has a white to yellow background colour with brown spots. Juveniles are less colourful. Its distribution is similar to its sister species, but is extended to New Guinea, Australia, Palau and the Fiji Islands. It is not as common as Gymnothorax tile. Gymnothorax polyuranodon can occur up to 30 km away from the coast. The maximum length found in literature is about 90 cm, though they do not seem to become larger than 50 cm following other studies.
The third species of freshwater moray eel more or less regular in trade is Echidna rhodochilus. It is among the smallest moray eels and only reaches about 35 cm following literature. It is distributed in Indonesia and the Philippines. The entire body is uniform brown or greenish. The head has a typical white, sharply demarcated spot at the corner of the mouth where upper and lower jaw meet. In contrast to that, Gymnothorax tile is rather grey than brown and its cheeks, which may be white, too, are not demarcated sharply from the body colour.
There is a number of further moray eel species, which can occur in brackish water as well as in fresh water and therefore sometimes are called freshwater moray eel, but they are not seen in trade regularly, if at all. The most prominent example is literally a slender giant. Strophidon sathete reaches a length of almost 4 m and is rather a case for a indoor pool than a usual tank.
Among advanced hobbyists it is known that all species referred to as freshwater moray eel in fact need brackish or marine water to thrive. The story is quite the same as with the brackish puffers T. nigroviridis and T. fluviatilis. They can last in freshwater even for months or years, but that is far away from the optimal care and thus should be avoided. The same is true for the freshwater moray eel. It's a shame that the word has not spread to many fish stores, which sell them as freshwater fish to beginners. Morays in fact can be successfully kept by beginners, but only if appropriate guidance is offered to them and the fish stores are still the major source for information of most customers.
All moray eels kept in fresh water share a similar sad fate. Although very hardy in brackish and marine water, they are prone to a number of diseases in fresh water. It is common for them to reject food and to starve within a few weeks, although other specimens manage to decimate their community tank mates before being weakened from various infections and parasites.
Why are they sold as fresh water then? The reason is quite simple, they were caught in fresh water. The importers noted that and forwarded it to the fish stores. That is how the legend of the freshwater moray eel started. The sticking point is that in nature they do not stay in freshwater. As to some extent euryhaline species they can and do make trips into the rivers for food, maybe to get rid of salt water parasites, possibly even for spawning, but they return to higher salinities within weeks. Stories like 'they were bred to live in fresh water', 'they are used to fresh water' and so on are just plain wrong.
Their natural habitats are the tropical estuaries and coastal mangrove swamps. Salinity in these areas changes with the tides and the monsoon, but mostly is rather close to marine. That is why a specific gravity of at least 1.010 is needed to keep freshwater moray eel. A full marine environment may be even better (especially for the adults) and is, due to the use of live rock and skimmers, easier to maintain.
Gymnothorax polyurandon and Echidna rhodochilus may last up to two years in very hard fresh water. The more often sold Gymnothorax tile will last significantly shorter. In high end brackish or marine water, they have a life expectancy of more than ten years making that the only environment that can be recommended.
Successful long term care in freshwater is not possible. Only buy one, if you can maintain a brackish or marine tank.
In addition to the right type of water, some other parameters based on the natural behaviour have to be considered for long term care. The needs of all three species listed are quite similar.
Moray eels may be predators and they may seem to have a vicious look in their eyes, but be aware that they are very easily stressed. Moving decoration around, netting the fish and especially changing the tank are typical actions, which can result in a scared moray eel that won't eat for days or even weeks. So its good to reduce such actions to a minimum.
Morays are very curious at night and investigate every crack or gap for food or eventual hiding spots. Keeping a moray eel in a tank without a lid is a bad idea. It is important to hermetically seal all holes in the lid, because they also like to investigate any holes up there. It is frequently reported they ended up dead on the floor. If it manages to flee and looks dead, get a net as soon as possible and put it back in some saltwater. They are hardy and even after hours there is a chance for survival.
Besides their mostly nocturnal exploring trips freshwater morays are rather inactive fish even compared to other species of moray eels. They will sit in some cave, preferably a narrow gap, all day and watch curiously what is happening around. They will only leave their cave if they smell food, see something very interesting or if the lights are turned off. But even then, there is not much activity, five or six rounds in the tank, a look in every corner and that's it. Consequently it is important to offer them lots of narrow caves and holes. Plenty of reef rock seem to be the perfect choice. If you are using a marine tank, porous live rock would be even better. As substrate avoid sharp things like broken oyster shells and prefer aragonite sand.
Because they don't swim that much, it may seem, that they need only small tanks, but actually that cannot be recommended. Due to their predatory diet, they produce a lot of waste which quickly can raise nitrate levels in a small tank. A relatively large volume of water and adequate filtration in addition to regular water changes are needed to keep the good water quality needed for long term success. Tank size for one adult specimen of Gymnothorax tile should be at least 120 l (30 gallons), two should consequently have 240 l (60 gallons). As with all fish, more would be even better.
Many first time moray keepers have problems to get their freshly bought moray eel to eat. Morays are robust and can live for at least one month without food, but given they already spent some weeks in quarantine and the fish store without being appropriately fed, they may starve quite soon after being bought.
A good diet seems to consist of a variety of frozen sea food. Frutti di mare from the supermarket are a good choice and contain different marine organisms such as prawn, mussel flesh, fish and squid. They should be cut into pieces for smaller specimens and should be fed with a pair of tweezers to ensure it is actually eaten. Being nocturnal hunters, their eyesight is pretty bad and it may take a while for the moray to find the food. Due to running power heads the food particles are spread all over the tank and locating it with the sense of smell can be difficult. Always feeding at the same location will help the moray to find the food.
If the moray refuses to eat, but seems interested though, turn off the lights. In the dark they are much more confident to leave their caves. If this strategy also fails, you may want to try to dangle a piece of food on a cotton string before its eyes.
If neither feeding in the dark, nor dangling before the eyes work, there always remains the unpopular use of feeders, be it crustaceans or fish. Unquarantined feeders from the stores always carry the danger of introducing diseases. Feeders from your own brood tend to be safe. Crustaceans like small Procambarus crayfish and shrimp are taken in general. Guppies and mollies will also work and tolerate the higher salinity of the moray tank. As soon as the number of feeders in the tank decreases, you can try again to feed frozen food. Sooner or later all moray eels will dare the step from live to frozen food. Just be patient, it may take weeks.
Feeding is the key to a moray eels heart. At first you may need to feed it in the dark, possibly with dangling or feeders. Next step will be that it accepts frozen food from the tweezers. Then you may want to try feeding with the lights left on. After some time, maybe some months, the moray will start recognizing that there is someone in front of the tank which might have some food and it will come closer to confirm that. If you choose to feed at a specific time, the moray eel will already impatiently wait for you.
It is not necessary to feed every day. Younger specimens can be fed every other day and adult ones about twice a week. The food pieces should match the moray eels mouth, which may be much more than you thought at the beginning. They have problems to tear apart larger pieces and may leave them rotting in the tank. Uneaten food should be removed carefully.
Hand feeding is possible, but may be a bad idea. The mucous membrane of the mouth produces a weak toxin. Own experience with regard to moray bites are luckily not available. Reports on the effect of the bite of Gymnothorax tile vary greatly. While some did not feel much pain, but bled heavily, others reported a constant pain in the bitten limb that lasted for an entire day. Secondary infections may also be a problem. It probably is a good idea to consult a medical doctor if bitten.
Moray eels in general are very individualistic fish, each one seems to possess its own personality. Accordingly, they may react very differently to tank mates. Some hunt down any fish in the tank, others prefer frozen food and ignore living fish. The same can be said with regard to crustaceans. I had success with hermit crabs and Porcellanella, but lobster like crayfish and shrimps do not last half an hour.
Fish, which have been eaten in the long run include guppies, mollies, archers and adult scats as well as tangs and wrasses. Schapfer (2005) reports he only had success with rock cods of at least 30 cm and even those had bite marks from time to time.
Another possible tank mate coming to mind especially in high end to marine brackish tanks is the green spotted puffer Tetraodon nigroviridis and other common brackish pufferfish. However, there have been reports of moray eel dying whilst trying to devour a puffed puffer. So loosing both fish would be a worst case scenario.
Freshwater moray eels can be housed in small groups without problems in tanks of adequate sizes. They show no signs of intraspecific aggression. You only have to watch when feeding that each specimen gets its spare.
As a result, only a species tank can be recommended for a long term successful tank without losses. You may try tank mates, but be prepared to loose them. A cleaning crew consisting of turbo snails and hermit crabs may be the only other moving tank inhabitants, although Echidna rhodochilus may try to eat them. The hermit crabs are useful to reduce the times you have to put a hand into the tank for cleaning, which helps to avoid stress for the moray eel. In addition, it is possible in case you have a marine tank to keep a huge variety of sessile invertebrates, which can turn the tank into a real jewel case with a little dragon.
It can be concluded that freshwater moray eels are a hardy alternative to the larger strictly marine moray eels. They will blow you with loads of personality, look out for you, when you enter the room and surprise you with their fierce look, but shy temperament. If you can offer an adequate brackish or marine tank and a varied diet, freshwater moray eels are very special and entertaining pets, which will accompany you for many years.
Allen, G.R. (1991): Field guide to the freshwater fishes of New Guinea.- Christensen Research Institute, p. 42.
Kottelat, M., A.J. Whitten, S.N. Kartikasari and S. Wirjoatmodjo (1993): Freshwater fishes of Western Indonesia and Sulawesi.- Periplus Editions, 221 p., p. 10-11.
Paxton, J.R., D.F. Hoese, G.R. Allen and J.E. Hanley (1989): Pisces: Petromyzontidae to Carangidae.- Zoological Catalogue of Australia (7), p. 131.
Pethiyagoda, R. (1991): Freshwater fishes of Sri Lanka.- The Wildlife Heritage Trust of Sri Lanka, 362 p., p. 49.
Schapfer, F. (2005): Brackwasserfische.- Aqualog spezial, 80 p., p. 43-45 (in German).
Talwar, P.K. and A.G. Jhingran (1991): Inland fishes of India and adjacent countries. vol 1.- Balkema, 541 p. p. 79.