All Goldfish have a swim bladder that helps them maintain neutral buoyancy. Due to their shape fancy Goldfish have a compressed swim bladder this causes them to have many more problems maintaining their balance. To prevent balance problems try feeding a wide variety of food including frozen foods like Bloodworm or Daphnia. It is also worth soaking any dried food for a few minutes before feeding. This prevents the food from swelling up inside the fish and putting pressure on the swim bladder. If your fish already has a problem you can try Swim bladder treatment. This, when used in conjunction with salt is quite an effective remedy.