Are you certain the problem is bacterial? Flukes and even advanced cases of Cryptocaryon can also produce those symptoms. Any chance of posting a photo of the fish for folks to see? Also, it would help to know the history of the tank....
I would not dose a display aquarium with Maracyn - it is just Erythromycin, and tends to inhibit the beneficial nitrifying bacteria without really affecting most common marine pathogens. Maracyn II would be a better choice IF you decide to treat the whole tank. I do use Erythromycin at low doses (12mg/l) to reduce cyanobacteria )bluegreen algae) in tanks.
Jay
