Tetra, TEHT ruh, is a type of small tropical fish that lives in rivers of South America and Africa. Tetras are also called characins. There are nearly 1,500 species of these fish. Tetras are popular aquarium fish because they can be raised in captivity and have beautiful colors. Most tetras grow 2 to 3 inches (5 to 8 centimeters) long and live 3 to 4 years.