Flavobacterium columnare - most bad bacterium, are Gram-negative. Since likely it is Gram-negative, Erythromycin (Maracyn) should work. If the fish is eating, another preferred medication is food containing
Oxytetracycline.
Aeromonas ubiquitas*, another gram-negative bacterium, will often become a secondary bacterial infection in Flexibacter-infected fish and is thought to be the cause of death in many cases of Flexibacter infection. Therefore, treatment should also be aimed at this bacterium.
If you are already using kanamycin (Kanacyn) in the water to treat the Flexibacter, it will also treat the Aeromonas. If you are using either erythromycin (plain Maracyn) or tetracycline, then treat simultaneously with minocycline (Maracyn-Two) for the Aeromonas.
Both kanamycin and minocycline are antibiotics, which are considered not to be harmful to the biological filter, but at high dosages, kanamycin can harm the bio filter.
When it comes to aeration do not underestimate the effectiveness of stirring the water a couple of times a day or daily water changes, Be sure to remove all activated carbon when treating with antibiotics in the water.
I would like to see this one grow to big the big bully he was always meant to be.
* The latin for omnipresence is a contrived one, Aeromonas is something very real however.