As Fantailer has suggested, it sounds like it could be swim bladder. See if the pea idea works.
He is also right in pointing out that the fish need a bigger tank. When it comes to stocking fancy goldfish, you need to have at least 40 litres per fish. This is usually given as a rule of 20 gallons/80 litres for the first fish, then 10 gallons/40 litres for each additional fish after.
The reason for this is that not only do the fish produce a lot of ammonia, but they also need the space to grow.
Some goldfish varieties, usually the single tailed's such as commons, reach up to 12 inches. The fancy varieties can grow to around 6 - 10 inches depending on the breed, although I'm sure in some cases they may grow bigger, depending on the individual genes and growth rate of the fish.
Would you be at all able to upgrade the tank?
You could consider the options of looking for a second hand tank, or perhaps contact a local aquarist society that may have some spare tanks available for sale quite cheap, websites and local ad's may also be a good place to check for a bargain
I hope the fish get's better! Please keep us updated