whats wrong with my fish?!

I took Guy W's reply to me to the lfs with the pictures and bought "Quick Cure" which has malachite Green and, Formalin in it... within hours the white stuff had come off of my fish. It says I can medicate it for 3days then I have to do a water change, I think i'm going to leave him in the hosipital tank for the three days ( and still medicate ) but the white stuff is gone, what was under it lookes like one small circular blister? I don't think paracite medicine will take care a blister... any ideas why she would have one? Thanks again everybody for posting you really really helped my fish out.
 
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