Red platy white patch?!

[fishdok]

Hi i have recently purchased (about a week ago) 4 young red platy's , they're all doing fine but yesterday i noticed on one of the fish a patch of white it doesn't look like fungus to me , so just to be on the safe side i removed the carbon and added some general tonic to reduce bacteria etc , any ideas on what this white patch could be?

 

[meegosh]

Is it fluffy white? Where is the patch on the fish? Have you dosed any meds?

 

[fishdok]

Is it fluffy white? Where is the patch on the fish? Have you dosed any meds?

No its not really fluffy, its near the rear fin but still on the main body , and I've dosed general tonic , not sure if its a medicine though