I am sorry to break the bad news, but the symptoms experienced by your fish are typical of swim bladder disease. When a fish has this, it looses control of its buoyancy. Unfortunately there isn't really a lot you can do to help a fish with this condition.
I have lost one fish in the past who had this condition.
Doing a quite look on the internet I found a couple of possible remedies:
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* 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 data yet, but it's worth a try.
* 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.
For prevention, heres a couple of tips:
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What can I do to prevent swim bladder disease?
As always, the golden rule of fish disease is WATER QUALITY. If swim bladder disease does have an infectious cause, your fish will be better able to resist this infection (and others) if your water quality is good. Regular water changes and water testing are a must.
Pre-soak your flake or pelleted food. This will allow expansion to occur prior to the fish eating it, and will lessen the chance of impaction.
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Perhaps it might just be worth trying something as mentioned above. I hope you have sucess in your treatment of this fish's ailment.
Kind Regards,
Matthew
P.S. Have a look at this google search I did on swim bladder disease:
http://www.google.co.nz/search?hl=en&q=goldfish+swim+bladder&meta=