A tetra is one of several species of small fish from Africa, Central America, and South America belonging to the biological family Characidae and to its former subfamilies Alestidae (the “African tetras”) and Lebiasinidae. Many of these, such as the neon tetra (Paracheirodon innesi), are brightly coloured. Tetra is short for Tetragonopterus, a genus name formerly applied to many of these fish, which is Greek for “square-finned” (literally, four-sided-wing). Tetras are toothed fish. They can live for 5-10 years. On average Tetra fish grow from 1.5 to 2.0 inches in length when they attain adulthood. The male of the species is slender than the female, as the female fish are a bit plump around the belly. They are very social and usually love to hang around in large schools. Tetras generally have compressed (sometimes deep) fusiform bodies and are typically identifiable by their fins. They ordinarily possess a homocercal caudal fin (a twin-lobed, or forked, tail fin whose upper and lower lobes are of equal size) and a tall dorsal fin characterised by a short connection to the fish’s body. Additionally, tetras possess a long anal fin stretching from a position just posterior of the dorsal fin and ending on the ventral caudal peduncle, and a small, fleshy adipose fin located dorsally between the dorsal and caudal fins. This adipose fin represents the fourth unpaired fin on the fish (the four unpaired fins are the caudal fin, dorsal fin, anal fin, and adipose fin), lending to the name tetra, which is Greek for four. Credit: Wikipedia.