Piranhas

The most common aquarium piranha is Pygocentrus nattereri, the red-bellied piranha. Piranhas can be bought fully grown or as babies, 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 rare to find individuals with one eye missing due to a previous attack. Piranhas are likely to become cannibalistic on others in their group if underfed.
Legendary reputation
There are various myths about piranhas such as how they can dilacerate a human body or cattle in seconds. These myths refer specifically to Pygocentrus nattereri, the red-bellied piranha. A recurrent myth is that they can be attracted by blood and are exclusive carnivores. A Brazilian myth called "piranha cattle" states that they sweep the rivers at high speed and attack the first of the cattle entering the water allowing the rest of the group to traverse the river. These myths were dismissed through research by Helder Queiroz and Anne Magurran and published on Biology Letters. Nevertheless, a study in Suriname found that piranhas may occasionally attack humans, particularly when water levels are low. Attacks on humans are usually reported around docks where fish are frequently gutted, and entrails are commonly thrown into the water.