Help with treating unknown disease in freshwater tank?

Have you tried feeding a medicated food ? That may be the way to go. Also try adding some PimaFix and MelaFix. This will treat any bacterial and fungal infections the fish may have that you are not aware of. Try raising the tank temperature to about 86, but do it slowly. One raise it about 1-2 degrees per hour. The source below may help you out if you look in the knowledge base and illness sections.