Loss of colour, loss of lip and red gills!

Okay so I noticed one of my mickeymouse platys was sat on the bottom, up until this morning everything was fine seemed normal but all of a sudden ive noticed its changed in colour, water testing shows up fine, no other fish affected in tank, the mickey mouse has now been taken out and put into a 12 litre quarantine tank and being treated for finrot and bacterial infection (as its the only medicine I had readily to hand) okay so stats and pics below

240 litre (53 gallon) (62 US gallons)
Ammonia - 0
Nitrite - 0
Nitrate - 20
Ph - 7
Temp - 24 celsius

 

Have her gills always been red? It may just be her coloration that allows you to see the redness.

 

No, her body was a beautiful blue and never saw any redness previously, there is another 2 and I cannot see any redness with them, even when I tried catching her shes didnt even attempt to swim away

 

That's weird. I'm sorry, I don't have an idea of what it could be

 

I guess keep treatment going and see if it effects them at all, hopefully I don't lose her