As other people stated it is important for us to know what kind of Ps you have, but since you mentioned piranhas not piranha it suggests you have more then one meaning you have fish of the pygocentrus group.
These fish do grow very fast for the first year. Mine went from nickle sized to 7" in as many months. Here is what I do.
I never feed my fish feeders (goldfish, ruby red, etc) because those type of fish have thiamese (sp?) which inhibats growth. Also I feed my fish a mixed diet with most of it consisting of frozen uncooked shell on shrimp. Nighcrawlers, catfish nuggets, and whole frozen baby blugill rounded out the rest. Ive heard beefheart wasnt good for them so I have stayed away from that as well.
The nightcrawlers I "clean out" before I feed them, what I do is start from the head and squueze down and squirt all the dirt out their ass so it doesnt make a mess in my tank. The catfish and whole bluegills are free since I like to fish but I freeze them for a few weeks to make sure there are now living parasites.
Another important factor is a powerhead. I purchased on that was recemmended for larger tanks then mine. I feel this current keeps them more active, which in turn keeps their metabolism up causing them to eat more and grow more!
Wow that was a long post!