Adults have a single row of interlocking teeth lining the jaw. True piranhas have tricuspid teeth, with a more pronounced middle cuspid or crown, about . Most adult piranhas - depending on the species - eat fruit, seeds, and fish. Some have been found with bird fragments, snakes, and small mammals in their stomachs. Young piranhas may eat copepods, crustaceans, and insects. Piranhas may feed more aggressively when their normal food resources become scarce.
Piranhas range in colour from yellow to steel-grey to bluish to partly red to almost black. Piranhas are normally about 15 to 25 centimetres long (6 to 10 inches), although reported individuals have been found to be up to 41 centimetres (24 inches) in length. Piranhas have a bulldog-like face with a very large lower jaw and many razor-sharp teeth. Piranhas in the wild are generalists, feeding on fish, crustaceans, insects, plants, fruits, nuts and seeds. Do not feed your piranhas live goldfish or other feeder fish if you can help it! They can transfer diseases and are not nutritionally balanced. Avoid buying adult piranhas .
Piranhas will often scavenge, and some species such as Serrasalmus elongatus are specialized scale-eaters, feeding primarily on scales and fins of other fish. Scale- and fin-eating is more widespread among juvenile and sub-adult piranhas. Piranhas lay their eggs in pits dug during the breeding season and swim around to protect them. Piranha fish belong to the Serrasalmidae family and are related to tetras, silver dollars, and pacu. They are multiple piranha species, and the common one in the aquarium is the red-bellied variety. Larger species average at around 12 inches while smaller ones reach a mere 4 inches.