There isn't much info I can give you really on Pirahna, as I am a newbie to keeping them. They are still very small at the moment and are in a tank with danios and minnows, but are starting to nip their fins. There are also some shrimp in with them.
I do know that they don't like much light, so there are lots of hiding places in there.
I have some driftwood floating on top of the tank as well that they love hiding out under.
They are also eating most flake foods (they don't like the cheap stuff) and love blood worm / live tubiflex...
I have fed them cockles as well, which they demolished