I think he just had a bacteria or parasite and he's fighting it off?
Could be either but if parasitic, the other fish would have shown some symptoms. Keep close watch on all of them. Continue as you have been and be sure to keep a close eye on the QT
water quality. If it remains so, a mild bacterial problem may dissapear on it's own. Personally I don't think it's bacterial though.
Was the QT
previously cycled or newly set up?
, parasitic infestations are there to stay unless treated properly. Fish can never be rid of most types unless the tank is fallowed.