As with all fish give them a varied diet. Pellets can make up the bulk of there food, just us several types, (IE growth, color enhancing, medicated, and general). Koi will eat fruits and veggies as well. Grapes, peas, zuchinni, corn, etc. Try worms for protein too. I've heard of people using hard boiled eggs as well, but just the whites. Pretty much anything fresh and healthy koi will eat.