Does it look like a cyst? If so then its probably a tumour or cyst (Follow my sig on diseases) If it is a tumor/cyst then there is nothing you can do except maybe feed him livefood for the rest of his life or euthanasia
I would consider lack of iodine too, because it usually causes "tumour" near gills.
And what becomes antibiotic, I wouldn't use them because they are need for human use - not for playing with them, it only causes resistant bacteria. (And I think antibiotic are not sold in Europe without prescripton, at least we need prescripton to get them.)