Knifefish

24 November 2017


Knifefishes come from half a dozen different families but all share a broadly similar appearance. They are somewhat eel-like in shape but with an anal fin that runs most of the length of the body from the abdomen to the base of the tail. In most cases the dorsal and pelvic fins are either reduced or absent. The caudal fin is usually reduced as well. Propulsion is generated by undulating the anal fin in a wave-like motion that works equally well for backwards movement as well as going forwards.

Most species are nocturnal, territorial and predatory, and most species get quite large. As such, they are fish for advanced hobbyists rather than complete beginners, even though some species at least can be considered quite hardy.

Taxonomy

Knifefishes can be divided into two basic sorts though: the electric knifefishes and the non-electric knifefishes. The electric knifefishes comprise five different families within the order Gymnotiformes and are exclusively found in tropical South America. The non-electric knifefishes are a single family with the Osteoglossiformes and are distributed across Africa and tropical South and Southeast Asia.

Electric knifefishes (order Gymnotiformes)
• Apteronotidae, 60+ species (including the black ghost knifefish)
• Gymnotidae, 30+ species (including the banded knifefish and electric eel)
• Hypopomidae, 16 species
• Rhamphichthyidae, 15 species
• Sternopygidae, 30+ species
Non-electric knifefishes (part of the order Osteoglossiformes)
• Notopteridae, 10 species (including the clown knifefish and African knifefish)
The species available in the aquarium trade come primarily from the families Apteronotidae, Gymnotidae, Notopteridae and Sternopygidae; only very occasionally are knifefishes from the families Hypopomidae and Rhamphichthyidae ever seen.

Basic care

Most of the knifefishes come from thickly vegetated areas in lakes and rivers, and appreciate tanks with plenty of cover. Thick stands of plants, either real or plastic, should be a feature of the knifefish aquarium. Rocky caves and piles of bogwood could also be used to create the required hiding places. Some species like to dig, so a substrate of silica sand rather than gravel is desirable. Floating plants are a definite plus, helping to make these fish more secure by cutting out some of the light.

Almost all knifefishes are nocturnal, so even in a shady aquarium you will not see them much of time, at least not when initially imported. They are often very sensitive to sound, and don't like tanks placed near loud television sets or banging doors. But once settled down most species become quite tame and can be weaned off live foods and onto frozen substitutes.

Knifefishes are usually intolerant of their own kind and predatory towards fish small enough to swallow whole. Many species are also quite territorial, sometimes aggressively so. Consequently they are not really community fish despite the fact that many of them are often sold as such, in particular the black ghost knifefish Apteronotus albifrons and the African knifefish Xenomystus nigri.

Feeding

Most knifefishes are predators. Small species feed extensively in insect larvae and other small invertebrates, while the larger species eat fish as well as frogs, water mammals and large invertebrates such as snails and crayfish. Under aquarium conditions it is best not to use feeder fish because of the risks of malnutrition and the introduction of parasites. However, newly imported knifefish may not take frozen foods immediately, in which case "weaning" them onto alternatives may necessary.

Useful foods for this initial training stage are earthworms and river shrimps. Small knifefish will also take live daphnia, brine shrimps and bloodworms. Once the knifefish is settled down and associates the keeper with food, it is a lot easier to introduce novel food items. Frozen bloodworms and mosquito larvae are the preferred staples for the smaller knifefish, and will be enjoyed by juvenile specimens of large species too. Bigger fish can be trained to take lancefish, whitebait, cockle, prawn, and other chunky seafood morsels.

Apteronotidae

Two species from this South American family are reasonably widely sold, Apteronotus albifrons and Apteronotus leptorhynchus, known as the black ghost knifefish and the brown ghost knifefish respectively.

Apteronotus albifrons is the more commonly encountered species. Part of its popularity comes from its rather attractive colouration: it is jet black with prominent white bands around the tail. Maximum size in aquaria is about 30 cm, though wild fish get considerably larger. It is also relatively well behaved when kept in a community tank with peaceful fish of comparable size. While it isn't overtly predatory, feeding primarily on worms and small crustaceans, large adults are perfectly capable of eating neon-sized tankmates. In common with most other electric fishes, Apteronotus albifrons is territorial and intolerant of other electric fish, including its own species.

Maintenance of this Apteronotus albifrons is not particularly difficult, but it is sensitive to poor water quality and needs a tank with a strong filter and regular water changes. Water chemistry is of secondary importance, though slightly soft and acidic water conditions would be preferred. Like most other knifefish Apteronotus albifrons needs a tank that is shady and provided with plenty of hiding places such as piles of bogwood or rocky caves.

Apteronotus leptorhynchus is similar to Apteronotus albifrons in terms of care, but can be distinguished by its mottled brown appearance and slightly smaller adult size. Both species are carnivores and appreciate small frozen and live foods including bloodworms, brine shrimps, daphnia and tubifex.

Gymnotidae

Probably the most infamous member of this South American group is the electric eel Electrophorus electricus. This species is most often seen in public aquaria, but it can be a rewarding animal for the home aquarium. The main issue is its large adult size and potentially dangerous electric shock. Adult electric eels get to a length of over 2 m, and obviously these will need to be kept in appropriately large aquaria. Oddly, adults are docile and quite tolerant of one another, but juveniles are notoriously snappy. Regardless of their behaviour towards one another, electric eels cannot be kept with any other species.

Maintenance is relatively straightforward. Newly-imported fish prefer live foods such as earthworms and river shrimps, but once settled down will take chunky frozen foods including prawns and whitebait. Water chemistry is not critical, but water quality should be good. Electric eels are air breathers and will drown if they cannot gulp air at the surface comfortably. Despite their reputation, electric eels will not shock their owners once settled down. The strength of the shock increases with size, and large adults are capable of pulses sufficiently powerful to stun a man. Obviously such fish should be handled with respect.

The knifefishes of the genus Gymnotus are commonly called banded knifefishes on account of the attractive colours of many species. The banded knifefishes are not as big as electric eels, but are still quite sizeable fish getting to a chunky 40 to 60 cm in length depending on the species. They feed on a variety of prey in the wild, including fish, shrimps, insect larvae, worms, and even some plant material. As such, they're quite easy to accommodate in aquaria. Live earthworms and river shrimps make good foods for settling in new specimens, but once tamed, they can be easily weaned onto frozen substitutes such as bloodworms.

The most commonly traded species is Gymnotus carapo, but the tiger knifefish Gymnotus tigre is a very beautiful species that has recently appeared on the market. None of the Gymnotus species is completely peaceful and all are intolerant of one another. They are best kept on their own or with robust tankmates of comparable size, such as large thorny catfish or plecs.

Notopteridae

Among the African and Asian knifefishes, the most widely traded species are Xenomystus nigri and Chitala chitala. The African knifefish Xenomystus nigri is a relatively small species that gets to about 30 cm in the wild but no more than 20 cm in aquaria. It is uniformly pinkish-brown to grey colouration. Compared with many other knifefish it has an arched rather than straight body and quite large eyes. African knifefish differ from all other members of their family in not having a dorsal fin.

Young fish are sociable, and this often encourages aquarists to keep them in groups, but they become increasingly intolerant of one another as they mature, and adults are best kept alone. Their modest size means that they can be kept with other fish including large barbs and characins, but even juvenile specimens will quickly eat small fish like neons or guppies. As such, they can't really be considered community fish despite often being recommended as such.

The clown knifefish Chitala chitala is the other species widely traded. It is a definite "tank buster" getting to well over 100 cm in the wild and frequently more than 60 cm under aquarium conditions. They are extremely powerful fish that become increasingly territorial and aggressive as they mature. They are of course predators as well, and given their large size will simply view smaller tankmates as food. Most specimens are kept on their own, but in a really big tank they can be combined with pacu, arowanas, big thorny catfish and so on.

On the other hand, clown knifefish are very attractive animals in a quirky sort of way. They have silvery-grey bodies with a series of black spots circled with white running along the flanks. Body shape is similar to that of Xenomystus nigri, being flattened from side to side and with a highly arched back. But unlike that species, the clown fish has a proportionally longer head and an altogether more solid, muscular build. Clown knifefish are highly valued as food fish.

Very occasionally other members of this family turn up in aquarium shops. Papyrocranus spp. come from Africa but have a longer body and less arched back than Xenomystus nigri. They are aggressively territorial and reputed to be fin-biters, so are best kept on their own. Notopterus notopterus is also sometimes seen. It is similar to Xenomystus nigri in terms of looks apart from the fact it has a dorsal fin. Aquarium care is similar to Xenomystus nigri.

Sternopygidae

The glass knifefish Eigenmannia virescens is the only species in this quite diverse group that is regularly traded. In many ways it bucks the knifefish trend in terms of aquarium, being relatively small, gregarious, non-territorial, and (largely) non-predatory. Maximum size in aquaria about 20 cm. They live in groups with a rigid pecking order, so keep a decent number to prevent bullying. Unlike many of the other knifefishes, they are not particularly predatory though bite-sized prey like livebearer fry will certainly be eaten. On the other hand, adult fish, even tetras, should be perfectly safe.

Glass knifefish prefer to eat mosquito larvae and midge larvae, and frozen substitutes in the form of bloodworms make a good staple food. Daphnia and brine shrimp are also enjoyed. Water chemistry is a little more important with this species than it is for the larger knifefishes, and soft, slightly acidic water is recommended. Filtration through peat is helpful. Water quality needs to be excellent, and again unlike many other knifefish, this is a species of fast flowing rivers rather than swamps, so water current should be significant.

Species from the families Hypopomidae and Rhamphichthyidae tend to be similar to the Sternopygidae in terms of basic care, though they are not necessarily gregarious and may in fact be territorial. Steatogenys elegans for example is known in the trade as the elegant knifefish and is a member of the Hypopomidae. It is an attractively patterned animal a little larger than the glass knifefish and not as gregarious, but otherwise similar in terms of care. While very common in the wild, it is hardly ever traded.