10:1 water:chlorine bleach works well for Columnaris disinfection. Boil your filter media or toss it. Keep in mind Flavobacterium columnare iis ubiquitous to freshwater, but if it were the only fish in the tank I'd probably do the bleach thing. In the display tank I'd up the water changes and maintain immaculate conditions to get the fish healthy and stress free as possible. I used to sterilize the first couple times some of my fish got it, then learned that it's basically a waste of time, IME. I'd still try to sterilize a hospital tank, though. Fish carry it, water has it, it's there. You just have to stop it from becoming pathogenic.
