i thought i would not comment, but no body is imforming you. there are certain fish that posess enzymes or chemicals that harm fish that eat them. goldfish and some others in the family, have a chemical(enzyme i believe) called thiaminase. it breaks down vitamin b1, and eventually causes negative effects on animals that injests it. so it slowly causes malnutrition.
also goldfish are nutritionally poor themselves i think.
so choose your feeders wisely. good luck
remember to try to help people on these forums