According to Goldfish as Pets, a variety of seafood, vegetables and meat can be given to goldfish in moderation. Goldfish can also eat freeze-dried foods, such as brine shrimp, and live food, such as earthworms, along with regular flake food.
Thawed seafood, such as oysters, clams, mussels, shrimp, lobster and other fish are all ideal foods to give goldfish. Other foods, such as beans, lettuce, spinach, peas, potatoes, broccoli, cauliflower, ground beef and egg yolks can also be given to goldfish. However, these foods should be given in moderation and used to supplement a regular flake food. Any live food provided to goldfish should come from places such as pet stores and not from places like ponds to help reduce the risk of introducing diseases into the tank.