Red-bellied piranhas (Serrasalmus nattereri) are from the Amazon River. They belong to the family Serrasalmidae. They formerly belonged to the family Characidae, which includes the neon and cardinal tetras, but piranhas were reclassified.
Piranhas are very aggressive fish and cannot be kept in a community tank. They will attack and eat the other fish. Piranhas have very sharp teeth and strong jaws. In fact, a group of piranhas can strip a large mammal down to the bone in a matter of a few minutes. There have been numerous horror movies depicting the feeding habits of piranhas. If you keep a piranha in your tank it has to be the only fish in it, or you can have a species tank and keep a few piranhas together. They are typically found in groups in their natural environment.
The piranhas you see for sale in pet stores are usually juveniles. The red-bellied piranha can reach an adult size of 11-12 inches (28-30 cm) in length. You should keep this in mind when selecting a tank size to house a piranha.
Like all fish from the Amazon, they do best in soft, slightly acidic water. The water temperature should be about 79°F (26°C).
They are carnivores (as mentioned previously). Their diet consists of fish, meat, insects, and worms. It isn't necessary to feed them live fish or meat. Although reports of their predatory nature is somewhat exaggerated, it is wise to use caution when putting your hands or fingers into the tank.
They are egglaying fish and have been bred in captivity, but it is somewhat difficult. Sexing piranhas is also difficult because males and females are identical in color. Sometimes females will have a more rounded body shape than the males if the female is full of eggs. You are more likely to have success with breeding piranhas if you filter the water through peat. Make sure you buy plain peat and not peat with additives that may be harmful to your fish. Strangely, piranhas don't eat their eggs like most other fish species do. The eggs will hatch a few days later and when they are free swimming they can be fed newly hatched brine shrimp.