Ok seriously.. I dont know much about p's but I know they get pretty big and need lots of swimming room.. but I'm sure the experts will be by shortly...
if you got a 1-2 inch serra species, you could keep it in there for about a year., then youd def need a much bigger tank. there are a few species that can live life in a 30 gallon though
You can use it a holding tank for your feeder fish. That is way to small for any piranha, I currently have a single red belly in 35 gallon tank all alone and I think it will out grow the tank in a few months.