sick platy?

[00steppie00]

I am not sure what it is so I am not sure how to treat it. She had little white spots that I thought were ich so I started treating her for ich but now it has spread and looks like white cotton on her body tail fin and one arm fin and it seems to be eating away at the fins???

 

[GNarnian]

To figure out what this is, we need to know a couple things. First, what size tank, what other fish are in there, and what are the water parameters?
Ammonia?
Nitrite?
Nitrate?

When was the last water change, and how much was changed?