The Knife fish, also known as Apteronotus Leptorhynchus, they are a species that belong to the Apteronotidae family, and that are native to rivers and lakes of the continent of South America. Although their name is knife fish, many know them as the black ghost, and they have been very fashionable for some years, so it will be very easy to find in specialized pet stores.
These animals are characterized by having a black body with a pair of white stripes at the end of their body, but it is their typical shape that makes these little fish very attractive animals. In the same way, it is quite peaceful although it will become a voracious predator of small tetras.
It is important to note that these little animals can measure up to 50 centimeters in length, making it a completely unsuitable fish to have in a home aquarium, unless you have a pond large enough to keep it there, although I recommend that you do not have in the home but rather in exhibition facilities and in zoos. In the same way, these animals are not very social, so it is advisable to keep a single specimen of the species to avoid territorial problems.
The right aquarium for these animals will be about 30 gallons of water, when they are young, while when they are adults, we will need at least one of 55 gallons. Don't forget that these fish feed on insect larvae, with small crustaceans, dry food and even small worms.