How long have you had the fish? Does it show any other signs of illness? Is it being picked on or otherwise being stressed? Usually Ick only effects fish that have weakened immune systems. But since one fish is showing it it's in there and since you don't need to treat the tank with medication, I would just raise the temp to 83-84 and do partial water changes frequently and vacuum the gravel really good to get all the ick spores out. Keep the temp up for 2 weeks with vacuuming and you should be good. Raising the temp speeds up the ick spores lifecycle and kills it quickly.
I highly recommend that you treat the entire tank for ICH and not just the fish you have in Quarantine. If one fish is infected the entire tank is infected. If your fish appears to have been sprinkled with salt then chances are it is ICH.
Although water changes are wonderful for your fish, it will not cure the ICH.
Jaysee, I'm glad it looks like you don't have a huge infestation of ich.
Gonna throw my thoughts out there...
That particular fish may have been weakened for some reason and less able to fight off illness, whereas (for whatever reason) the others were able to keep ich at bay and you caught it in time.
Another is ich can be in the gills where it can't be seen, or
It wasn't ich you were seeing.
Could be wrong on all accounts, but thought I'd share.
I'm only guessing here, but it could be that the one fish had a more compromised immune system so it wasn't able to fight off the infection where as the other fish were. Perhaps the other fish had at some time been exposed to ICH and overcame it. When fish have successfully overcome ICH they build up a higher resistance to it. Hopefully the rest of them stay ICH free. Good luck!