Get it out and into a seperate tank and start treating with a broad spectrum medication (wunder tonic / melafix etc.) (unless you can pinpoint the exact cause) and/or salt. Daily water changes. No food for a few days (fish can happily handle this - once they're better they'll eat). Watch the rest of the tank for any other signs over the next few days / weeks.
Those are my thoughts anyway...
Some people will say don't treat with medication unless you know what you're treating but I figure the fish is getting worse by the hour anyway so the worst thing that will happen is the fish is a gonner, best case it gets better instantly in a clean, well filtered tank with the broad spectrum meds treating the right thing.