Tetra is the common name that is given to the small freshwater characiform fishes. It is a type of fish that is very small in size and is the prey to many predators. There is no accurate number as to how many tetras there are in the world. Tetras are known as schooling fish. Some of the known varieties are Buenos Aires tetra, emperor tetra, black tetra, Mexican tetra, bleeding heart tetra, penguin tetra, bucktooth tetra, x-ray tetra, vampire tetra, blue tetra and more. They live mostly in groups and are found to be happy and stress-free when they are in their respective groups. When keeping these fish as pets, you should have at least five to six tetras so that they can play with each other in the tank as a group, or else they will get lonely.