White fuzzy spot on healthy platy?? Help

[janice_016]

I have one platy that has a white spot on her, she is healthy and last time i treated the tank with mardal maracyn the fish with a white spot died. so when i saw a white spot on my other platy i decided to just treat with a little extra salt in the tank. thought it was working and the white spot started to fade.
Now i see my other platy has a white fuzzy spot all of a sudden. And one of our fry just died. Tank is fully established. All levels are good and steady and have been for over a year. Do regular water changes.
I try to avoid chemicals as much as possible. Should I just up the salt a bit? Any idea of what to do.
I'm just about to add 4 guppy fry I got from a friend. They are floating in the tank now, ready for their separated net area in the tank

 

[TidBitz]

My featherfin cat had a white spot that looked kinda like a fungus without the fuz, due to the delicate skin I used melifix and it worked great. Bout all I can tell you without a picrure.