nothing. No qt, no meds, just added a uv sterilizer after about a week of him looking like this. It took about two weeks for him to finally fight this off and get better. In that time, he pretty much parked himself in front of a koralia. He continued to eat like a pig and swim normal. There were about 9 or 10 other fish in the tank with him and none came down with it either. This was almost 3 years and 2 tank upgrades ago. No other outbreaks have occurred despite the stress induced from the upgrades.
This is why i advocate a "wait and see" approach rather than the "yank and medicate" approach so many around here advocate.
I don't think my experience is all that unique.