if its Brook then that is why I posted the link it tells you what you have to do it is a very good artical and formalin is right but it could be flukes and prazi-pro will work very efectivly also on a few other prosibilities. I had a strane if flukes a while back that were transferable used prazi-pro worked very well. lost 2 fish 1 from stress trying to catch and QT and a second that had nothing todo with this outbreak.
As I said and others have said you need a clear picture of the spot.
Bill