@Bill: Mate, the primary cause of any disease in fish is stress. By using energy to deal with the stress factors, their immune systems don\'t have the energy available. Look for anything that might cause stress and remove it. Overstocking, air fresheners in the room, draughts... anything. The best way to look after a fish tank is with benign neglect, lest you become a stressor yourself.
For right now, a 33% water change and a dose with Myxazin (without the charcoal, of course). Then set about finding the stressors.