Hi,
What is recommended is sinking pellets that are soaked for a few minutes in some tank water. I ordered Progold from Goldfish Connection. It is supposed to be the best food for goldfish. And the sinking pellets keeps them from swallowing air that could cause swim bladder disorders.
www.goldfishconnection.com
My fish just love them. I use a small bowl, put some tank water in it, a pinch of pellets, let it sit for a few minutes and the pour it in the tank. They dive down and have a feast. I feed them twice a day.
About once a week, I take a frozen pea, take the skin off and cut it into about 8 pieces and drop them one at time into the tank.
I have some freeze dried blood worms that I also soak and feed occasionally. I haven't tried other veggies or fruit.
My fish have doubled in size and I bought them on July 4th of this year.