There should never be a main food that you give them, a variety is the best because piranhas are scaengers in the wild so they eat all kinds of stuff, not to mention it will help them grow faster and they will have more brilliant colors. I would just get something different evry week. Fish fillets from the supermarket is ideal, you can always switch it up for them. My reds like catfish fillets, frozen shrimp, and salmon fillets the best.