I've been having a similar problem, in my 10 gallon tank I currently have 2 Platy and 2 Guppy (each different sex for type of fish). My female Micky Mouse Platy just wants to sit at the bottom of the tank, I know she can swim fine but all she does is lay at the bottom on the gravel with her tail fin at an angle. And my male Sunburst Platy has a swollen gill on his right side (left if you looking at his face) because i was moving him from a different container and I accidentally hit him while trying to get him with the net. It is definitely swollen compared to the other one and it a little red too. Also just to mention, my female guppy is currently pregnant and in a breeders net. But what can I do for both of my platy?