The most common aquarium piranha is "Pygocentrus nattereri", the red-bellied piranha. Piranhas can be bought fully grown or as young, often no larger than a thumbnail. It is important to keep "Pygocentrus" piranhas alone or in groups of four or more, not in pairs, since aggression among them is common, not allowing the weaker fish to survive, and is distributed more widely when kept in larger groups. It is not uncommon to find individual piranhas with one eye missing due to a previous attack.