I think ur tank and stock will benifit from a waterchange!!! Fungus/bacteria can be a result of dirty conditions!!! It is suggested to preform a weekly wc, do you test ur water??
Anyway, buy dechlor, siphon out 50% water and add fresh water mixed with dechlor around the same temp, do this every week and in a few weeks ur fish will be prefect!
Also some salt may help, 1tblspn per 10 gals will do
Well somethings not fine cuz ur fish is infected, unless u got him recently, it got sick in ur tank! It could be from stress but I doubt it, what do u mean ur params are fine? Is the tank even cycled??
Columnaris...Salt dip(see attached link. the concentration to treat ich won't do anything for it) can clear it up if it is only on the body...but antibiotics is called for if the infection is systematic.
Lower the temperature, or the disease will act faster.
If the problem persists, or unwilling to do consider destroying the fish to prevent it spreading.