Have you considered trying an extended freshwater bath using tapwater? Tapwater contains Chlorine and possibly other disinfectants. My guess is that the otherwise healthy CBB would hold up to a properly prepared tapwater bath better than whatever is in his gill. I once cured an otherwise healthy Regal Angelfish of a bad case of pop eye with this method. Popeye is usually a bacterial infestation. Freshwater baths are not indicated for bacterial diseases. However, my thinking was that by using tapwater, I incorporate the tapwater’s disinfecting properties. It took four baths but I saw marked improvement after each bath. Something to consider