Sorry to pull up an older thread, but I wanted to thank Karlas for my newly bookmarked link. Along the same lines as the one above....here is one of the links I keep on Fish Disease. Scroll down a little, then you pick the area that is of concern (i.e. skin disorders, eye disorders, kidney, fin, etc etc). Then you are taken to a chart of symptoms that you follow along answering questions to get to the diagnosis and subsequent treatment.
Hope it helps someone out!
Fishyfarmacy