Glass fishes come in all shapes and sizes, originating from South America, Africa, and Asia. The glass knifefish, Eigenmannia virescens, is one of the more common species from the Amazon basin and can be safely kept in a community tank containing medium to large fishes.

Glass knifefish care

These amazing fish glide around the tank effortlessly backward and forwards as if the water they live in provides no resistance. When they are observed close up it is possible to see the fishes’internal organs, which are neatly squashed into the first quarter of the body. This has given rise to their other name, the glass knife fish.

E. virescens is a nocturnal species that prefers the company of others. However, if provided with a densely planted tank with plenty of hiding places they usually don’t mind being on show during the daylight hours.

Being a gregarious fish, I would suggest that you keep a group of at least six. They often take part in slight spurts of aggression although injuries are seldom if ever, encountered.

Most species of knife fish with a total body length of 45cm/17″ would never be considered for the large community aquarium due to their predatory lifestyle. This fish, however, is a notable exception and can be kept with other medium to large peaceful fishes including gouramis, large tetras or loricariids. In fact, basically anything with a peaceful disposition.

In the wild, these fish find their food by the use of a weak electrical field around their bodies which can sense the most minute changes in their environment. Food consists of tiny fishes, bloodworm and other small aquatic life.

These fish have a large anal fin which runs almost the entire length of the underbody, a dorsal true caudal fin are absent. AKA as the green knife fish, they are often imported with what look like small black oval things inside the clear muscle tissue of the body. I can only assume these are the egg or larval stage of some sort of parasite which remains dormant in captive fish. They do not appear to be harmful.


  • Green or Glass knife fish
  • Eigenmannia virescens
  • Pronounced: Eye-gen-man-ee-ah vye-ress-ens
  • Family: Sternopygidae
  • Size: Males are larger growing to 45cm/17″, females are usually much smaller reaching around 25cm/10″
  • Origin: Widely distributed across the Amazon basin.

Aquarium care

Provide at least a 90 x 38 x 38cm/36″ x 15″ x 15″ for a small group. Any type of filtration will do; however, if you decide to use undergravel filtration use a powerhead rather than an air pump to power it.

This is because these fish come from flowing bodies of water so they will appreciate the extra current a powerhead delivers.

Water parameters: pH 6.0-7.0 with a GH 2-15, temperature 23-28ºC/75-82ºF. In some parts of the range, this species is found in the water of just 18ºC/64ºC.

Diet: Food consists of tiny fishes, bloodworm and other small aquatic life. Aquarium raised fish can be fed frozen brine shrimps, bloodworms, daphnia as well as flake and sinking granular foods.

Breeding the Glass Knife Fish

These fish have apparently reproduced in captivity. To stimulate the breeding process an increase in the water level followed by a drop in pH and temperature reduction simulates the rainy season of their natural habitat.

The fish give off electric signals during courtship. The discharge of electricity from the male is said to allow the fish to locate potential mates of the same species in murky waters and provides a cue to spawning.

Experiments in aquaria involving simulating the electric signals produced by males has been shown to trigger courtship displays in female Glass knife fishes.

Around 100-200 eggs are laid during the early morning with the dominant male fertilizing the eggs of more than one female. These are usually deposited on floating plants, many hundreds of eggs are laid if two or three females are present.

Alternatives: There are eight species in the genus Eigenmannia, including E. nigra, E. humboldtii, E. limbata, E. macrops, E. trilineata, E. microstoma and E. vicentespelaea. However, these are rarely imported intentionally although you may find them among shipments of other species if you look closely.



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