A large fish shaped like an everyday knife is commonly known as the knifefish. This fish has an elongated look and are kin to another group of fish called eels. Most knifefish come from the fresh waters of South America, but a few species live in Africa and southeast Asia.
The knifefish fits into two orders, the Gymnotiformes and Osteoglossiforms. These orders are made up of several sub-families, including Apteronotinae, Electrophorinae, Gymnotinae, Hypopominae, Rhamphichthyinae and Sternopyginae. These families inhabit different parts of South America in the fresh waters. There are several species of South American knifefish that typically range from 8 to 24 inches (20 to 61 cm) in length.
Some knifefish live in Africa and southeast Asia. These are from the Notopteridae family. This family is similar to the South American families except that the caudal and anal fins are merged, showing a larger fin. These species range from 12 to 40 inches (30.5 to 101.6 cm) in length.
Many of these knife-shaped fish from South America can be raised in a home aquarium. The ones that can mature in a tank include the black ghost, brown ghost, speckled, banded, green and glass variations of knifefish. The fish from Asia and Africa that can be raised in an aquarium include the clown knifefish, Asian knifefish and African knifefish.
These fish can be fairly large, so they need to live in a tank that can hold at least 90 gallons (341 l). Knifefish prefer to be in a darker space with plenty of rocks and plants under which they can hide. They can be injured by sharp-edged rocks, though, because they do not have scales.
Most of these fish are nocturnal, but some can be active during the day. When being raised in a tank, they will feed in the dark. They eat live food such as insect larvae, worms and crustaceans as well as chopped meat. Some of these fish will jump out of the tank if the tank lid is not securely closed.
Knifefish can be known to be aggressive toward each other or toward fish of a smaller species, and they will eat smaller fish in the wild or in captivity. In a home aquarium, they do best with fish of similar size and growth. They usually are not aggressive toward fish that are medium to large in size. Most species of knifefish live three to seven years, with a few species living to be 10 years old.