my serras do the same thing to live feeders. but silversides get completely eaten.i'd steer away from goldfish and rosey reds, due to parasites(unless you quarantine them for a week or 2. and due to the fact that cyprinid fish are deficient in thyamine. try to switch him to tilapia, shrimp etc. he'll eat it it just might take a few tries before he'll accept it.