Well Goldfish need a bit more variety than just flakes.
Try feeding Pellets (Such as pro-gold from goldfishconnection.com)
Feed them veges aswell such as:
Kiwi
Orange
Peas ( They love it and are good for them try feeding them this once a week)
cucumber
apple
lettuce
carrot
green pepper
cabbage.....The list goes on. just soak them in water and make them into bit size pieces.
You can also get these foods from your LFS.
Shrimp
Tubfix worms
Krill
blood worms
A variety of these foods will really show in the color and healthness of your fish. Such as color will become more vibrant, They will be more active, they grow nicely (If given enough space, filtration, and areation aswell). Just an over all healthy goldfish!.
I feed my goldfish twice a day, as much as they can eat in 2-3 minutes.
I also Fast them on sundays, this will cause them to clear their systems and give them a break. They will also look for food in the gravel and clean up the tank for ya a bit so it will keep them busy for a day!