What kind of piranha is it? If its a black piranha you can only house it alone! Notthing, meaning no other fish can be put with it or everything will be killed cept the black piranha itself.
A fish diet is VERY VERY bad diet. Best thing for any species is to get get them acclimated to a pelleted diet. But if there red bellies, yes they hunt in packs of 3 or more and like someone suggested like dim water. Floating plants can help. They really are a shy species unless feeding.
In the past, I've seen piranha's take to Cichlid pellets, squid, krill, earthworms, freeze dried whole shrimp all tied down to a rock with a rubberband cept the pellets.
Also for future reference, RB piranha's get 12-13 inches and are messy feeders often leaving chunks of food under the gravel so over filteration and 10% water changes once a week is my opinion to keep ammonia and nitrite levels low.
~How long are your piranha(s) and what kind? Most pet stores have a selection in according to a specfic size, L,M,S-Coments, Tuffies(rosies), Guppies. If you just got it, give it a few days to settle in. Make sure water quality is 100% and temps are in the high 70's. I'm guessing your tank already went through the Nitrogen Cycle
before putting the piranha in? If not, be contious of over feeding.
Sry for long post and added info prolly as which you already know
Have fun! There an underated species of fish.