Be careful over treating with a lot of different products-the Betta can get toxic..and you need to be mindful of the Balance- both good and bad bacteria-since these products can't tell the difference-you can sometimes make things worse by killing everything both good and bad-often the good bacteria is what can help keep the bad bacteria under control-this can also disrupt antibody development and sometimes over treating can mask important signs/symptoms.
The bacterium responsible for columnaris-Flexibacter is ever present-or always in the water and no amount of cleaning, bleaching...etc....will rid it from the tank-its the fish immune response that keeps it from infecting....Stress is the leading cause that can compromise the immune response-not just water quality issues but sudden changes in temp and chemistry that can stress a fish-you can also create an environment that is more suited for the bad bacteria and cause higher than normal levels in the system.....
You also have Aeromonas and Mycobacterium
The best treatment I have found for this-High dose aquarium salt 3tsp/gal, tannins, water movement or high dissolved oxygen and water changes.
You have two strains of this-one that will usually kill the fish in 12-24h that eats the flesh away and one that the fish can survive-this usually presents itself in the form of fuzzy white-off grey area on/about but not limited to the head, mouth, gills and back-often looking like a saddle.
You want to be careful boiling rock and gravel-since they can explode...
While bleach is not really needed and won't do much for some bacteria-any residue can be taken care of by dechloranating products.
Saprolegnia spores are always in the tank feeding on organic matter-waiting for a break in the tissue, immunosuppressed to infect the fish along with Achyla another fungi both being opportunistic as most fungi.....