If a fish, platy in this case, doesn't show any obvious sign of disease - then there isn't much you can do other than check the water parameters and temperature to make sure they are in the ideal range.


Some things you could check are the pH , water hardness, and temperature. Ideally, the pH should be around 7-8.5 with hard water and the water temperature on the cooler side around 73-77 Fahrenheit (23 -25 Celsius).

Also you can check to see if she's pregnant, since this can alter her behaviour. A pregnant female platy will tend to hide a lot more then usual and sometimes sit on the bottom or in a corner somewhere. It's always a good sign when a fish continues to eat.

Just thought I'd update:
After keeping them in an emergency tank while treating Horus, Isis started swimming happily with the rest of the platies when I introduced them to their home again after cleaning it all thoroughly to get rid of all the germs
So no more problems on my side