artificial crab meat would be a big no-no, it is made with fish, usually pollock, and tons of artificial ingredients to give it more of a crab look and taste... you definitely want to stick to raw shrimp and fish fillets and pellets.
when choosing a healthy diet for your piranhas, it is extremely important to keep in mind that their natural diet consists of 97% fish and fish parts, their digestive systems handle this diet best and it is the same diet that you should provide for them in your aquarium.