google search the clamped fins... youll find loads of expert advice about it. i know there is an illness which prevents fish from swimming properly, as if they had broken their spines. Is the fish constantly disfigured? this can be caused by stress! research it though, type in, fish illnesses and stuff like that etc.
Check ammonia and nitrite levels to rule out water quality problem, you might has an islolated case of Skin flukes, does the platy in question have a grey looking appearance and breathing rapidly?.
Dont leave it any longer as disease will kill if not treated in time.
I'm pretty sure it is not skin flukes because she has normal breathing and has nothing wrong with her on the outside. She does become very isolated sometimes but does occasionally swim around normally especially when it is time to feed.