You normally only see Piranha in a tank to themselves. In fact, I've never heard of anyone trying to keep them in a mixed species tank. Then again, I'm not a fish person. I guess they must be very well fed.
Which begs the question - how do you feed them without them getting into a frenzy and attacking the other tank inhabitants?
The Smithsonian National Zoo also exhibits Piranhas in mix tank habitats with other larger and smaller fish species. It is the second freshwater tank in Amazonia, their indoor mini rainforest.
It's relatively common to find piranha mixed with smaller fish in England. In London Zoo, for instance, they are in with X-ray tetra, and in the London Aquarium, they share the tank with Butterfly Goodeid.
I think that the different species of piranha have rather different temperaments, but many of them will live reasonably peaceably with other species if the tank is big enough. On the other hand, those big Cichla would probably snap up some of the smaller fishes in that tank if they ever started to feel a little hungry.
