No idea without pictures. On the safe side, I'd quarantine him and treat with antibiotics.
With columnaris you never want to increase temps as it thrives at higher temps.sounds like columnaris (flexibacter infection).
its a b*tch to treat. took me over a week to cure a betta of mine of it, and bettas are really hardy to begin with.
quarantine the fish, but treat everyone. its contagious as hell.
i used sulfathiozole, but kanamycin would probably work better. increase temps, add salt, the works.
With columnaris you never want to increase temps as it thrives at higher temps.
Treating with sulfa based gram negative antibiotics along with about 2 teaspoons of salt per 5 gallons of water in the tank.
You can also do a short term bath with methalane blue, salt and an antibiotic such as maracyn 2. The bath is done for 30 minutes.
Here is some great info on columnaris.
But. Realize you would have to use a microscope to identify it.
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