Fancy goldfish require greens in their diet so some blanched broccoli and deshelled peas would be good. They do better on sinking pellets if you don't have them already. Defrosted frozen bloodworms or brine shrimp can be fed occasionally but not everyday like greens and pellets.
Well, I have four fancies. They eat broccoli as Mumma mentioned. Day before tank maintenance I sometimes do a slice of orange or lemon. I also do brine shrimp the frozen packs you get at your LFS. And I make my own gel food up for them.