Gymnotiformes

The Gymnotiformes is an order of teleosts which are electric fish. Common names are Neotropical knifefish or South American knifefish. They are mainly nocturnal freshwater fish. Their muscles create an electric field. 

Gymnotiformes are slender fish with narrow bodies and tapering tails. Their common name is "knifefish".

Perhaps the best-known species is the electric eel (Electrophorus electricus) which uses powerful electric shocks (up to 600 V) for hunting and self-defense. 

There are several other species familiar in the aquarium trade: the black ghost knifefish (Apteronotus albifrons), the glass knifefish (Eigenmannia virescens), and the banded knifefish (Gymnotus carapo).

 References