A Piranha or piraña  is a member of a family of omnivorous freshwater fish which live in South American rivers. In Venezuelan rivers they are called caribes. They are known for their sharp teeth and an aggressive appetite for meat. However, despite the negative Hollywood publicity piranhas are not generally violent, and have been known to be domesticated in home and office fish tanks.

Piranhas are found only in the Amazon basin, in the Orinoco, in rivers of the Guyanas, in the Paraguay-Paraná, and in the São Francisco River systems; some species of piranha have extremely broad geographic ranges, occurring in more than one of the major basins mentioned above, whereas others appear to have much more limited distributions. However, piranha (inevitably former aquarium-dwellers) have been introduced into parts of the United States, even being occasionally found in the Potomac River, but they typically do not survive the cold winters of that region. Recently a piranha was caught by a fisherman in the Catawba River in North Carolina.  This is the first known case in North Carolina and possibly in the region. Piranha have also been discovered in the Kaptai Lake in South-East Bangladesh. Research is being carried out to establish how piranha have moved to such distant corners of the world from their original habitat. It is anticipated that rogue exotic fish traders have released them in the lake to avoid being caught by anti-poaching forces.

Locals 

Locals often use piranha teeth to make tools and weapons. Piranha are also a popular food, though if an individual is caught on a hook or line it may be attacked by other piranhas.

Piranha are commonly consumed by subsistence fishermen and often sold for food in local markets. In recent decades, dried specimens have been marketed as tourist souvenirs. Piranhas occasionally bite and sometimes injure bathers and swimmers, but truly serious attacks are rare and the threat to humans has been largely exaggerated.  A piranha bite is sometimes considered more an act of carelessness than that of misfortune. But piranhas are a considerable nuisance to commercial and sport fishers because they steal bait, mutilate catch, damage nets and other gear, and may bite when handled.

Pets

Several piranha species appear in the aquarium trade. Piranhas can be bought as pets in some areas, but they are illegal in much of the United States, such as in Washington State, New York, Utah, Nevada, and all Southern states from the West Coast (California) to the East (Florida), and up the Northeastern seaboard.
 
The most common piranha is the Pygocentrus nattereri, or the red-bellied piranha. Piranhas can be bought fully grown or as babies, often no bigger than a thumbnail. It is important to keep Pygocentrus piranhas alone or in groups of three or more, not in pairs, since aggression among them is common and is distributed more widely when kept in larger groups, allowing the weaker fish to survive.

When kept in groups, it is recommended that they are in even-numbered groups, as piranhas will gang up on an odd member. It is not rare to find individuals with one eye missing due to a previous attack. The preferred feeds are thawed shrimp, fillets of white fish, and disease free feeders, but any fish-based foods are adequate. The young are to be fed very little, as overfeeding can kill them. Blood worms, or insect larvae are a good choice of food, as they are full of protein.
 
If underfed, piranhas are likely to become cannibalistic on others in their group. They eat more as they grow older and larger. In order to provide a balanced diet, it is usually necessary to change types of food often. Feeder goldfish are a popular choice for feeding piranhas, although they contain a vitamin B inhibitor that may stunt growth and shorten the fish's life span. It is recommended to feed them with feeder goldfish as a treat, once in a while, rather than basing their diet only on that. Piranhas prefer a darker environment with a lot of plant cover, as they become agitated when denied appropriate cover. It is not advisable to leave the light on constantly, for with too much light, they may lose the desire to eat.