I have a couple of platies in a 10 gallon tank that I got from a friend
Now, I have four ladies and two boys, all but one lady, Isis are very active swimming about.
Thing is here, she doesn't show any kind of illness whatsoever, her fins are all erect and she eats well (even if she doesn't come to the surface to do so), I change their water regularly etc.
Do you have any ideas on what's wrong? She's the biggest one in the bunch too... : D