PGBRAT,
I found this snipit via a Google search.
SWIM BLADDER DISEASE
The swim bladder is the organ which allows a fish to stay at any level in the aquarium without sinking or floating. The swim bladder may fail from damage by bacteria, parasites, genetic faults, or blows and/or bruises. When the swim bladder fails to function the fish loses it's ability to swim normally and may swim sideways or even upside down. Once damaged, the bladder does not usually return to normal functioning, but if the fish can eat and swim without too much strain it can live for years with the condition. Goldfish seem particularly prone to swim bladder problems. Without knowing the exact cause of the malfunction, treatment is difficult. Since internal bacteria, fungus, or parasites are the only treatable causes, medicated food and/or antibiotics should be tried along with frequent water changes.
If the other fish have not been affected, then I wouldn't suspect a rampant problem with bacteria/parasites in your pond water, but I would keep a close eye on them just to be cautious. I would suspect that this one fish may have had a developmental or genetic disorder, a one time infection or got hurt at some time, but will be fine. He just limps a bit now.
I would keep him quarantined for a while longer to monitor him and the other fish. If he does alright, I'd probably put him back in the population eventually. Or maybe put a friend with him in the same isolation tank first and see if the friend develops the same symptom. If it doesn't, then it is probably safe (not a communicable disease or parasite/bacteria).
I am certainly not a fish doctor, but this sounds like a reasonable approach to figure it out.
Gordy