Sick sunset platy

[gump clipper]

I have a 24'x12'x12' tank with a plec,7 danios and 2 sunset platys and 5 tiny baby platys. I have been doing regular waterchanges and havent added any fish recently, yet one of the platys is not moving much and seems to be getting white fungus on its fins, has any one any ideas? what is the best thing t do?

 

[gump clipper]

Its died now but non of the others have the symptoms, have found a platy baby today so its true when people say the just reprduce without a mate, every thing else in the tank seem fine.