Pacu, PA kyoo, is the common name for several species of South American freshwater fish. Pacus have a narrow, plate-shaped body. Some pacus look much like piranhas, to which they are closely related. However, unlike piranhas, pacus have blunt teeth and eat plants and plankton (small, drifting water organisms). Piranhas have sharp teeth and eat fish. Some small pacus are popular for home aquariums.