he needs to be quarantined (sorry lol) and i used erythromycin (a broad spectrum antibiotic) before with success..
do you have a hospital tank you can put him in for a bit? i tried maracyn plus, and it didnt do anything so i switched to erythromycin. Now, there is a maracyn two-that is an antibiotic, and you can put it in the tank with everyone, and it doesnt crash you biological bacteria.. ive never tried it, so i dont know. Maybe others here can tell you more about that.
ampicillin is another broad spectrum antibiotic that should work. add salt also.
id quarantine ( if possible) and treat with emycin and if that doesnt work after about 5 days do a waterchange and try maracyn two..