I think you first have to figure out what the disease is. If it is bacteria or fungus, this is usually caused by poor water quality, and cured by better filtration, skimming and larger water changes. If it is ick (oodinium) then you're dealing with copper treatments and this is when you need to keep your tank fallow. I would read on fish diseased on wetwebmedia.com and don't do anything before you know exactly what you have and what you're gonna do for it, because I can tell you from experience that there is a fine line between treating you fish with copper and poisoning them. Good luck.