External Treatment for Columnaris in Fish

[Carlrs]

"Merbromin is a lessor known product to many newbies in the hobby due to difficulty in obtaining, but to those who have been in the industry and hobby for a long time its benefits are well known.
In fact this is a product NO SERIOUS AQUARIUM KEEPER SHOULD BE WITHOUT, especially when one considers its low costs and benefits

Merbromin (AKA Mercurochrome) is a topical antiseptic often used for minor cuts and scrapes in people. Merbromin is an organomercuric disodium salt compound and a fluorescein.
It is useful on external infections because of its permanence, and lethality to bacteria, in particular Columnaris."

http://www.aquarium-medications.com/2017/10/aquarium-fish-merbromin-external.html