Sounds like swim bladder to me-
1. Feed your fish a couple of peas. That's right, peas. Just get some frozen peas, thaw them, and feed them to your fish. A professor of fish medicine at N.C. State College of Veterinary Medicine has done this in several cases with very good results. He thinks that the peas somehow encourage destruction of the impaction. No hard scientific evidence, but it's worth a try.
2. Fast your fish for a couple of days. Withhold all food for three or four days, and sometimes this alone will break up the impaction and return things to normal. Most fish can go a week to ten days without food and be just fine.
3. Periodic aspiration of the swim bladder works very well. Basically, you stick a needle in the swim bladder and suck out some of the air. Not something to be entered into lightly, but does work well. This is not a cure, but a successful treatment.
Some of these treatments seem a bit extreme but it is worth giving a couple of them a try before going to the vets. There is also a Partial pneumocystectomy. This is another word for surgical removal of part of the swim bladder. But this is for the Vetnary side of things so is probably out of the question unless you know a vet that has performed such a procedure. (Very rare, heard about it through a Goldfish breeder).
How old is the other Goldfish, if he is young he may just be developing his adult colouration. I have had Goldfish take up to two years to turn from black to a gold colour.
