Table of Contents
- Types of Knifefish Species
- Black Ghost / Phantom Knifefish (Apteronotus albifrons)
- Clown Knifefish (Notopterus chitala)
- Transparent Knifefish (Eigenmannia virescens)
- Asian Knifefish (Notopterus notopterus)
- Brown Ghost Knifefish (Apteronotus leptorhynchus)
- African Knifefish (Xenomystus nigri)
- Aquarium Care
- Knifefish Food
Knifefish are an ancient group of fish that has the ability to generate a weak electrical field around their body. They are primarily located 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 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 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.
Types of 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 / 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.
Black Ghost Knifefish Size
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 toward 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.
Black Ghost Knifefish Water Parameters
All specimens are wild caught, and they do best in Amazonlike water conditions. A recommended pH would be acidic, at about 6.5, while water hardness should be on the softer side at 80 to 150 ppm. The black ghost will, however, adapt to varying parameters of water chemistry, as long as conditions remain relatively stable, and the temperature is kept in the range of 72 to 76 degrees Fahrenheit. I have kept this fish successfully in slightly hard water with a pH value of 7.4.
Feeding Black Ghost Knifefish
The black ghost has nocturnal tendencies, but one of the most rewarding aspects of keeping this knife over others is its proclivity to recognize its owner or at least the hand that feeds it. The fish eagerly accepts live foods, including small worms and brine shrimp. You can try holding a block of freeze-dried Tubifex worms at the aquarium surface, and if the fish is comfortable in its surroundings, don’t be surprised if you find it coming right to the surface for a nibble. Despite the electrical organ, there is no shock to the human hand, as this organ is for sensory purposes only. I have received many “oohs” and “aahs” from witnesses of this feeding event.
Clown 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.
Clown Knifefish Size
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 formidable creature sports a series of large fingerprintlike spots that sit equidistantly from each other on the fish’s steel gray body. The placement of these spots can vary from individual to individual but generally begin just ahead of the dorsal fin, going back toward the tail.
One thing that enhances a hobbyist’s urge to purchase clown knives is that they are sold at somewhere between 3 and 8 inches — much smaller than their adult size. The smaller versions may initially lack the spots, but they will emerge as the fish grows. The smaller clowns can sometimes be housed in a community aquarium and might even be a peaceful tankmate. As they mature, however, they will attempt to eat anything that fits into their capacious mouths. As youths, they will accept standard aquarium foods, being fond of live, frozen or freeze-dried worms. As they grow, so too does their appetite, especially for live foods.
Clown Knifefish Tank Requirements
To exploit their growth potential and impressive looks, you want to keep them in at least a 55-gallon (48-inch) aquarium. Originating from the sometimes murky, swampy waters of Thailand and Burma in Asia, these fish are not overly fussy about water conditions. They are comfortable in temperate water on the acidic side of the pH scale (6.0 to 6.5).
Clown Knifefish Tank Mates
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.
If you are looking for a tankmate for an oscar fish, Jack Dempsey or other large aquarium fish, the clown knife could be just the right companion. They certainly can handle themselves and will usually not bother like-sized fish.
Clown Knifefish Breeding
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.
Transparent Knifefish (Eigenmannia virescens)
Another commonly sold knifefish from South America is Eigenmannia virescens. This most tapered version of knifefish is commonly called a transparent knife or green knife. It has a translucent quality that may have a tint of green on its body. As for the ghosts, I have enticed these knives to surface for hand feeding. This will happen most often with a scheduled feeding regimen and some wormy foods to nibble upon (freeze-dried Tubifex blocks are ideal).
Transparent Knifefish Size
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.
Transparent Knifefish Tank Mates
These 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:
The transparent knife, unlike other knives, prefers to live in a community that includes other members of its species. 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. A social hierarchy will develop, and the fish will honor this system in the aquarium. They will, however, become quite intolerant of pesky barbs and tetra fish. Other smaller, less intrusive fish can be kept with them, but keep a watch as to what is happening in the aquarium. The fish that do not cross over into knife territory are generally left alone. You’ll probably need at least a 4-foot aquarium to keep more than one knife with other community members.
Transparent Knifefish Breeding
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.
Transparent Knifefish Water Parameters
As for water parameters, this species can do well in water toward the higher end of the acidic scale, but going past neutral (7.0) can be deleterious to this fish’s health. This particular fish does best with frequent and small water changes. Changing 10 percent once a week is better than 30 percent or more once per month. This will keep the pH level the most stable, ensuring these fish do not succumb to a widely fluctuating pH.
Asian Knifefish (Notopterus notopterus)
Asian Knifefish Size
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.
Asian Knifefish Tank Mates
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.
Brown Ghost Knifefish (Apteronotus leptorhynchus)
The brown ghost knifefish (Apteronotus leptorhynchus) is similar in appearance to the black, except for its coloration, which is a coffee color with white dorsal piping. The nose is more angular and is sometimes referred to as a long-nosed knifefish. Aside from their general appearance similarities, their personalities vary significantly.
In my experience, the brown ghost is not eager to ascend to the surface for feeding, but would rather remain on the bottom, jutting out to grab a morsel of fish food as it slides by. Others have found this species to be on the delicate side and that it is more timid. Hiding places within the aquarium are vital to this species’ health. Long pieces of driftwood with openings (caves) serve this purpose amply.
Keepers of brown ghost knives need to be vigilant in observing their specimens, as they are prone to fungus. They are shy creatures, and any pesky or curious tankmates may be stressful to them, compromising the effectiveness of their immune system. This knifefish is really not for beginners but rather for advanced hobbyists willing to dedicate a species-only aquarium to this fish. More innocuous tankmates, such as cory cats, can be kept with a brown ghost; and the ghost, in turn, is not aggressive toward other aquarium inhabitants.
Both the black and brown ghost will want to stay hidden in the aquarium — the brown more than the black. You can provide hiding while still easily seeing them by tricking their electrical sensory system. Simply place a clear tube in the aquarium. They will feel hidden, and thus remain visible without knowing it. It’s best to keep only one of either species in an aquarium, as they tend to bicker amongst themselves. I recommend that you try keeping the black ghost first and go on to the brown after you have successfully kept the former.
African Knifefish (Xenomystus nigri)
We’ll switch continents to examine another frequently available knifefish — one of the duller-looking specimens. The African knife (Xenomystus nigri) is native to West Africa and lives in still, slow-moving waters with dense vegetation. They are widespread in the coastal river basins of Sierra Leone, Liberia, Togo, Benin and Cameroon, as well as in the Chad, Nile, Congo and Niger Rivers.
African Knifefish Size
African knifefish have been known to reach lengths of 8 to 12 inches, though more frequently they are offered for sale at a juvenile size of about 4 inches. As they grow, they will become much more precocious than the black or brown knives. They are avid predators in the wild and will surely behave the same in your aquarium, especially as they grow. In nature, their diet would consist of worms and insects found on river beds. This can be duplicated at home with frozen and freeze-dried varieties of the same. They rarely eat flakes or pellets.
African Knifefish Water Parameters
As to water quality and chemistry, normal tropical temperatures are fine, but keep in mind that their natural waters are on the acidic pH side (in the 6.0 to 6.5 range). The African knife will most often begin foraging the aquarium for food close to dusk, as they are nocturnal. Provide hiding places, and as they become comfortable in their new confines, they should come out in the open to feed.
African Knifefish Tank Mates
Even as juveniles, the African knife can be pugnacious. Do not keep this fish with smaller, more peaceful species. Keep them in the company of larger cichlid-like fish that will not succumb to threatening behavior.
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.
Absolutely No Metal in the Aquarium
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 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 be 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 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.
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.