Order Gymnotiformes
Suborder Sternopygoidei
Family Sternopygidae
Members of the family are characterized by a long anal fin (up to 240 rays, with no caudal or dorsal fins) starting in front of the gill opening, with the anus located under or in front of the eye. These are crepuscular carnivorous fish. They are susceptible to stress, toxicosis, and bacterial infections. pH 6-7.5; dH 2-15; T 23-27°C.
Genus Eigenmannia
— Glass Knifefish — Eigenmannia trilineata [Lopez & Castello, 1966]. (Eng. Glass Knifefish). South America, Paraguay and Paraná river basins. 22 cm. Active during the day species. Best kept in small groups, obligatorily more than 2 specimens. Only peaceful aquarium tankmates. Darkened aquarium with a cover of floating plants; they are nocturnal and quite timid. During courtship and spawning, they emit “electric” sounds. Good filtration and water pump flow are needed since fish like to swim in the current. Small live food. T 23-27°C; pH 6.0-7.0; dH 3-10°.
— Green Glass Knifefish. Eigenmannia — Eigenmannia virescens [Valenciennes, 1836]. Synonyms: Eigenmannia lineate, Sternachus virescens, lineatus, tumifrons. (Eng. Glass Knife Fish, Green Glass Knifefish). South America. Males 30-45 cm, females 20-25 cm. They have neither dorsal nor caudal fins; the anal fin has about 240 rays. The body is translucent, light green to light brown in color. Sometimes the flanks have a greenish sheen. Two stripes on the sides run along the transparent anal fin, and others are located directly above this stripe.
The head ranges from dark brown to pinkish-beige with large eyes. Fins are transparent. In adults, the anal fin has a black border along the edges. Males are much larger and slenderer than females. Fry have the same translucent, gray-bronze coloration with a greenish sheen as adult fish. A peculiar gill apparatus allows fish to breathe even under oxygen deficiency.
This fish has an electric organ that allows it to generate a very weak electric field around its body, which helps detect objects in the water, gives it spatial orientation, and helps locate prey. They locate food and conspecifics not only by sight but also using electrical impulses. The tip of the thin tail carries an auxiliary tactile function (if damaged, it regenerates quickly, just like in lizards).
A peaceful species that grows to large sizes. In a large aquarium, densely growing plants, driftwood, stones, flower pots, and pipes for hiding places are needed, along with a sandy bottom. Floating plants to reduce lighting. Must be kept in groups of 3 or more. A social structure will form a hierarchy with a dominant male and weaker individuals. Compatible with large peaceful species that they cannot swallow. Fish become active at night.
Their behavior in the aquarium is interesting. During the day, they usually stand motionless, hiding between stones or among plants. These fish have poor eyesight, so they use electrical impulses, a kind of radar, for navigation and object searching. Having noticed prey, they make a short dash and, having grabbed it, return tail-first, unerringly feeling their way back into the recently left shelter. This species prefers “old” water. Aeration, filtration, partial water changes.
These fish are mostly wild-caught and initially very timid, secretive, and difficult to feed with aquarium foods. Live food only: small fish, snails, crustaceans, worms, bloodworms, tubifex. With insufficient feeding, they begin to get sick. Reacts painfully to increased concentrations of table salt and medications. Mature at 2-2.5 years. Lifespan is 6 years. dH 2-15°; pH 6.0-7.5; T 22-29°C.
Identification: Glass Knifefish — Eigenmannia virescens.
Description of family Polypteridae, Erpetoichthys calabaricus.
Gymnorhamphichthys rondoni, Rhamphichthys rostratus. 
