Kanaplex has been my only success against columnaris.
I cannot guarantee you have this, but it appears that way.
I had NO success with changing ph or temps. I tried Triple Sulfa drugs, Furan, Furan2, Malachite green+salt (NoxIch), Erythromycin, PraziPro, Paraguard, Metroplex, almond leaves, & blackwater additive. Probably other things I cannot recall, and struggled with this for 6 mos in my tetra tank
I had never had or seen columnaris before, but when Kanaplex knocked it out, it was confirmed in my mind.
I believe that the Melafix may provide some topical relief to the fish (as do some other things) but I have been told that enough of that stuff to kill an established disease is fatal to most fish, and I doubt it can prevent re-infection.
One other thing to mention (If this is columnaris) is that one or two rounds of treatment plus repeated water changes can get it under control; but it is likely that four 6-day treatments may be necessary. This is expensive in a big tank.
The good news is, though I lost 4 of ten fish, 3 of the remaining who had nasty skin and crippling internal problems (presenting as spinal deformities!) all cleared up and the remaining 6 are healthy now for one year.
The bad news is that I literally spent HUNDREDS of dollars and dozens of hours treating $50 worth of fish in a 33g system.
Cost is based on how much water your tank holds. A 100g would cost 3x as much to treat.
I would have saved so much $$$ had I known what I had and what to do.