Fancy goldfish such as Orandas, Ryukin, pearl scale etc are prone to losing their balance. This often happens shortly after feeding and has always been blamed on them taking in air when they feed at the water surface. Whilst this may contribute to the problem in a few cases, I believe the main problem is feeding too much food.
Fancy goldfish have very short bodies in comparison to normal goldfish. This has resulted in the swimbladder (which provides buoyancy for the fish) being in very close contact with the other internal organs. If they are fed on too much dry food, it may swell when inside their intestine, putting pressure on the swimbladder and forcing it to one side of the body. The result is that the fish are unbalanced and may swim on their side.
To overcome the problem Fancy goldfish should be fed very sparingly. Give them as much food as they will eat within 1 minute and feed twice a day. Using a sinking food such as the FishScience Fancy Goldfish Pellets will help as it ensures that they feed from below the water surface. If a particular goldfish is prone to a loss of balance, try soaking the food before feeding so that it has already expanded before they eat it.
If all of the fish in an aquarium are affected, the issue may be due to poor water conditions and be nothing to do with the swimbladder. A large partial water change will help. Ensure the new tap water is the same temperature as the aquarium water, and the chlorine is removed with a suitable conditioner. Then find the cause of the poor water conditions. Water tests may help, but also check that the filter is working effectively and that the fish have not been over fed by accident.