I'm sorry to read about your struggle w/ Columnaris. It's more than just my opinion, aquarium specific antibiotics rarely cure fish. Just review the mountain of related threads, which document fish antibiotic failures.
Using antibiotics in the display tank will likely put a big dent in the
BB, or totally wipe it out. The resulting unstable
water parameters have the potential to stress the healthy tankmates, opening them up to infection.
IMO, best to remove any obviously infected fish asap and either euthanize or treat in a hospital tank.
After removal, a big 50% WC would be a good idea.
Hope things get better soon.