The array of adaptations that have evolved in fish has resulted in their ability to inhabit a wide range of different habitats — even if there’s water-only part of the year, there are usually fish. As fish hobbyists, we are fortunate to have a plethora of species to choose from that vary in shape, color and behavior.
One adaptation, in particular, is decidedly unique: the presence of electroreceptors. There are more than 60 species (representing less than 1 percent of all fish) that can generate low-frequency electric fields around their bodies. These external electrical fields are produced by specialized electric organs (modified muscles) and are used in predation, defense, orientation and/or communication.
An ancient group of fish that has the ability to generate a weak electrical field around their body is the knifefishes. They are distributed throughout the tropical waters of Asia, Africa, and both South and Central America. The common name was acquired owing to their knifelike shape, with most species lacking a dorsal, tail and ventral fins. Instead, they have an elongated anal fin that runs from just behind the pectoral fins to the end of the greatly reduced tail fin.
One might assume these fish are extremely slow because of the lack of the usual fins, but they can move surprisingly fast by rapidly undulating the anal fin. In fact, due to their unique morphology, these fish can move both forward and backward and orient themselves at angles that few other fish can replicate.
Because knifefishes inhabit murky water, they evolved small eyes and have poor eyesight. As a result, during daylight, they remain hidden in poorly lit areas among dense vegetation, driftwood, and any other dim, protected area. As the sun disappears to the west, however, knifefishes emerge and go on the prowl, using their special electric organs to “feel” their way through the water.
Producing a small electrical field around their body by contracting and relaxing specialized muscles, they are able to detect potential food or predators when the electrical field is disturbed. If the disturbance signifies food, the knifefishes will home in on it and attack with surprising accuracy. If a potential threat is determined, the knifefish quickly flees the area, while at the same time supercharging its electrical field to further confuse the predator.
Popular Knifefish Species
There is more than one type of knifefish. Let’s cover the species kept in aquaria here, starting with one of the most well-known species.
Black ghost or phantom knifefish (Apteronotus albifrons).
This is probably the most ominous-looking species because of its jet-black body. This dark coloration is only interrupted by a white patch on the head and two wide white bands near the base of the tail fin. The common name is in reference to South American natives that believe that the ghosts of the departed take up residence in these fish.
This is a large species at 20 to 24 inches. It does well in a large community tank, but it is very territorial and aggressive towards others of its own kind. Only one black ghost should reside in an aquarium. Other than this, its mild disposition toward other fish has made it popular with aquarists. It can be housed with peaceful species, such as large gouramis, cichlids, arowanas, plecos, and Oscars.
As a predator, it will prey upon any small fish, so select tankmates carefully.
Until a few years ago, all specimens offered for sale were collected from the wild and imported from South America, primarily Brazil. Currently, the black ghost knifefish is bred in Indonesia, eliminating pressure on wild populations.
Clown or Spotted Knifefish (Notopterus chitala)
The clown knife is native to Southeast Asia, including Thailand, India, Malaysia, Sumatra and Borneo. This fish is my personal favorite. It is the most striking knifefish, with a row of pale white or gold and black circular markings along the middle of its silver body. The color varies in some individuals. Some will display a more off-white or pale gold color. This may be the result of age, the surrounding environment or diet.
Large specimens (up to 36 to 40 inches in the wild) are truly magnificent. Like other knifefish, it is active at dawn and twilight, remaining hidden during daylight.
This species is aggressive toward other clown knives, and violence will ensue if more than one specimen is present in the same aquarium. It is, however, rather timid toward other large fish. This fish can be kept in a large community tank with other calm fish (e.g., peaceful cichlids, large barbs) large enough not to be preyed upon. A 55-gallon aquarium will suffice for specimens up to 10 to 12 inches, but in the long term, you will have to upgrade to a tank of 250 to 300 gallons — so be prepared.
The clown knifefish is bred on a commercial basis in Thailand, where they are also an important food fish. They lay eggs between May and July, and the male guards both eggs and brood. Some reports state that soft water and a lot of floating plants — which may be used as a substrate for the eggs — are necessary for spawning, though hard substrates are sometimes utilized.
The male guards the eggs until they hatch, with incubation lasting about one to two weeks. Upon hatching, the fry will accept newly hatched brine shrimp. It is suggested that newly hatched fry be transferred to a separate aquarium. I’m not aware of any records of successful breeding in home aquariums, and there is no apparent sexual dimorphism.
Glass or transparent knifefish (Eigenmannia virescens).
This is one of the smaller species, attaining a maximum size of 12 inches. The common name is in reference to the semi-transparent and slightly green-hued body, making the internal organs, skeleton and veins visible in young specimens. Specimens become more opaque as they mature.
This knifefish are shy toward heterospecifics yet social among themselves, which is why they should be maintained in small schools. They will coexist peacefully in a large community aquarium with other docile fish, such as platies, mollies, guppies, larger tetras, small barbs, Corydoras and so on. Unlike other knifefish, this species prefers the company of its own kind.
In nature, glass knifefish live in groups and have a well-established, organized social structure. Each member in a group of glass knifefish will produce an electric field at a different frequency from one another. This allows them to hunt and navigate together while not becoming confused by the electrical signatures of other members.
Females are reported to be smaller than males. Reports also state that this knifefish lays eggs among plants. Breeding can be difficult, with spawning usually encouraged by simulating rainy season conditions by doing large water changes. To raise the fry successfully, transfer the adults to a separate aquarium.
Asian Knifefish (Notopterus notopterus)
The Asian knifefish is slightly larger than the glass knifefish, which grows to 14 inches. Not as spectacular as the previous three species in coloration and markings, the body is a dull silver with the ventral surface exhibiting a light pinkish hue. They are aggressive toward their own kind and will readily consume any food that will fit in their mouths. Given this, they should be kept in a large community aquarium with other larger fish, including large barbs (e.g., tinfoil), silver dollars, Bala sharks and the like.
Although young specimens will live in small schools they become intolerant of each other as they mature and must be separated. There are a few reports of spawnings that took place at night and resulted in up to 200 eggs deposited on the bottom of the aquarium. Similar to the clown knifefish, the male guards the eggs until they hatch, and incubation lasts about two weeks. Once hatched, the fry will accept newly hatched brine shrimp.
Although the knifefishes discussed are native to waters covering many continents, their general care is similar. They can initially be housed in a relatively small tank (e.g., 30 gallons), but as they mature, I suggest upgrading to 55 gallons for both the Asian and glass knifefish, and 200 to 300 gallons for the black ghost and clown knifefish. If you are not prepared to provide larger systems, please do not consider purchasing these magnificent fish.
As noted in the descriptions, other than the glass knifefish, it’s best if only one knifefish is housed per aquarium. This will prevent any territorial and aggressive behaviors, as well as stress due to disruption of the fish’s electrical field. In addition, avoid other electrogenic fish, such as elephant-noses (e.g., Campylomormyrus tamandua and Gnathonemus petersi).
I’ve already noted recommended sizes and species for knifefish tankmates in community setups. As a knifefish grows, some of these smaller community fish may need to be swapped out for larger fish so that they do not become live food. Avoid aggressive fish, such as tiger barbs and some cichlids, which may harass and thus cause stress.
Probably the most important criterion when setting up a tank for knifefishes is aquascaping. Because knifefishes inhabit murky water, you want to create an environment with an abundance of dark or dimly lit areas using a variety of driftwood, rock and dense live or artificial plants.
If live plants are integrated into the aquarium, it will need relatively high-intensity lighting. Therefore, the overall aquascaping must provide the knifefishes with areas that are significantly shaded. Robust plants, such as swords and grasses, work well, with floating plants, such as duckweed, Salvinia and water hyacinth, used to assist in shading the aquarium.
In addition, a length of PVC pipe integrated into the aquascaping will be used by the knifefishes. A knifefish that feels exposed or insecure due to inadequate acquascaping may become stressed and stop feeding, and thus become vulnerable to disease.
No Metal for Knifefish
Keep in mind that the presence of any metal, particularly ferrous metals (iron, steel), can be a problem in the knifefish environment because the metals can cause problems with their sensory electroreceptors. Avoid metal thermometers and any other metal equipment in their setup. Running a strong magnet through the substrate to be placed in the aquarium is a good idea to detect and remove ferrous contaminants.
The location of the aquarium is also important. For example, I would not suggest a room in which overhead or other lighting is turned on and off at all times of the day. In addition, you might consider the use of moon or lunar lighting during evening hours. Also, knifefishes are scaleless, so there shouldn’t be any sharp edges on rocks, and more importantly, the substrate should have smooth round edges.
Excellent, consistent water quality is of the utmost importance for maintaining healthy knifefishes. An efficient filter is necessary to ensure zero levels of ammonia and nitrite, and frequent partial water changes are needed to maintain minimal nitrate levels. The filter’s return flow show is directed so that no significant currents are created.
Although knifefishes are considered hardy, poor water conditions will lead to their demise through stress and resulting in bacterial infections and fin rot. Water temperature should be between 73 and 81 degrees Fahrenheit, with a consistent pH on the acidic side (between 6.0 and 7.0); these fish are intolerant of alkaline pH values, and even more intolerant of pH fluctuations.
Sudden changes in water chemistry can make these sensitive fish prone to ich. If treatment is necessary, they cannot be treated using medications containing caustic dyes and/or metallic compounds (see the sidebar “No Metal for Knifefish”). For treating ich, I suggest using the elevated water temperature method or using zinc-free malachite green at half the recommended dosage.
Food for Knifefish
Knifefishes will thrive on a variety of commercially prepared, protein-rich flakes and pellets. Also include Tubifex worms, frozen and freeze-dried foods, and live, meaty foods, including bloodworms, crickets, and feeder guppies or goldfish.
Because knifefishes spend most of their time near the bottom of the aquarium, it’s important to ensure that sufficient quantities of food make it to the bottom. In nature, the larger knifefish species are primarily piscivorous, but if you start early, you may be able to wean them over to the commercially prepared foods just outlined. Don’t be surprised if some specimens only accept food during the night.
If you’d like to consider something completely new, different and interesting, knifefishes may be the answer — if you’re up to the challenge. Provide the appropriate aquarium that is aquascaped for them, choose tankmates carefully, and make sure they receive a balanced diet. The knifefishes will do the rest.
Jeff Howe holds degrees in biology and marine biology. He has worked at the Waikiki Aquarium and Smithsonian Institution, and is now a Fish and Wildlife biologist in Florida.