Because you should NEVER feed live fish to your fish. The only exception being fish that have bred in your care and kept in pristine conditions to avoid disease and even then it's a risk. Where did you get the feeders? Pet store feeders are riddled with disease and it could be that your fish is reacting to just such a bad feeder. Feeders are also not very nutritious and gold fish feeders prevents and important vitamin (B or D I think) from being absorbed into the fish. If you want to give your fish live food get worms, crickets, grasshoppers(my fav) ect... Also what size tank? Tankmates? Water parameters, ammonia nitrite and nitrate readings? Water change schedule?