Is It Really A Fungal Infection, Or Something Else?

I was told that one of my golden barbs had a fungal infection, so set up my old smaller tank as quarantine and was treating him in  there.  He'd been in there about three weeks and in the last week it looked to me like he was better.  The black markings were back on his scales and all the scales appeared to be there, though some of them did look a little paler than the others.
So I decided to put him back in the big tank.  All seemed fine.  Went out and when I came back, the other golden barbs were picking on him again - chasing him around and trying to go for where the spot was before.  When he swam in the right direction, the white patch was back and really quite noticeable.  It had been about 8hrs since he was put back in that tank.  I've fished him out again and he's back in the quarantine tank, which I've given another dose of fungal treatment just in case (thankfully I hadn't got around to emptying it).
Would a fungal infection have come back that quickly, if it hadn't fully gone before?  Is it something else?  This is the best picture I can get of him.