Fins and scales. In the wild I can see how it's possible, and the victim fish will swim away and may or may not heal. Doing this in an aquarium would probably be a bad idea though as they can't really get away. Just like any other species tho there diet can be supplemented artificially. A live food alternative pellet, such as the large floating cichlid pellets that say "live food alternative" should do them just fine I would think since its formulated for any carnivore including piranhas. Getting a wimple to initially eat that tho could be a challenge because as far as I know all wimples are wild caught and not accustomed to artificial foods
