There is some excellent frozen food in the pet store, like bloodworms, brine shrimp, krill. It comes a bit closer to their natural diet, and doesn't plug up their digestive system like flakes can. You can give them that as a weekly treat or something.
Spirulina flakes are also a great choice, and what I have seen in stores today was something called Goldfish bites. Its sinking pellets, and they contain vitamins and extra nutrients for the fish. Haven't tried those yet though....