The fact that only one of the fish in the tank is experiancing the problem suggests it is not something wrong with the whole tank, so I would preliminarily rule out water problems, plus you say water is normal - but what do you consider normal? But the fact that this sounds to be a quite sudden change in behaviour suggests it is something wrong with the tank. But if it is to do with the tank you can fix it, start by doign afew things avoxo said: and an air line, increase the temperature, make sure the food starts sinking before it is eaten so no air is taken in then, and you could try medication. Also carry out a series of 50% water changes over period of days, and this should start straighening out any water quality problems. However, if it is a natural (if you can call it that) deformity - which can be common in fancy goldfish as other organs press against the swimbladder, meaning it cant fill out properley causing the fish to have trouble with its balance. If this is the case, there is little chance of any 'cure'. But many fancy goldfish live perfectly happy lives like this.
Another method that can help with swimbladder problems is to keep the fish in shallow water, acouple of inches deep or so, and this can sometimes help fix the problem.
Hope it all helps,
Thom.