I have a planted tank. I have no choice but to treat the whole tank. as lots of my fish are infected. So a salt bath etc wont work in my case. I recently tested my water using my API mater test kit and all results came back good. My PH is around 7.5.Are you positive it's columnaris? If so, use an antibiotic that will kill gram-negative bacteria.
See if ciprofloxacin tablets are available in your area- if not, use a methylene blue dip for the fish. If you don't want to medicate/don't have anything yet, a salt dip may work.
Carry these out in a quarantine tank if you can since it will kill BB as well.
Lower your tank temperature to around 75F to slow the spread of the disease
Perform frequent water changes to reduce stress. (what are your parameters?)