Pacus have a reputation for being swimming garbage bins and there is nothing more true that that. Pacus absolutely eat anything and everything that it can gets in its mouth. Things that don't fit in its mouth, it has a set of reasonably sharp teeth to do the job. There is a widespread myth that pacus are vegetarians (herbivours) eathing nothing but veggies. That is not true! Pacus will eat fish and meat anytime, however, they usually prefer to prey on fish at night when the targets are asleep. They are not safe to be tankmates for prized fish like arowanas, because they would not hesitate to nip the fins of tankmates when the opportunity arrives.
Due to their excellent appetite, pacus grow big and fast especially when young. Only recommended for huge tanks.