help my platy

[FightClubChick]

ok, heres my problem. About a month ago i got 5 platies, all babies, and put them in my 5 gallon aquarium with nothing else in it. They were all living fine and very playful until about a week ago.
I came home and found one of my platies dead stuck on the filter. I hoped it was just one of those things that happen, tested my water, and it came out fine. Then in the past few days I noticed my largest platy, almost half an inch, hiding all the time and only coming out to eat, while the others still played together. Then today I came home and found him stuck against the filter, but still alive.
I helped him get free and could tell his fins had been nipped badly and he wasnt swimming very well. I moved him to a 2 gallon aquarium i set up a week ago that i was cycling. He swimming a little better but is still hiding like he had before, even though nothing is in the tank with him.
I don't know what to do now. I think he'll be ok, but I dont know if I should move him back or wait till hes bigger or what. They are still to little for me to tell sex so im not sure if that has anything to do with it or not. I dont know what to do or why they are doing that, and why they are just doing it now. Anyone have any suggestions?

 

[Feline]

Which filter r u using? Could the current be too strong for them? Or that the fin nip is actually fin rot where the fin's wasting away?

May be u can do a small water change n add salt to the tank to see if they get any better.

Edit:
Just saw on ur signature u have a rubber lipped pleco in the tank with the babies. how big is it? N how big are ur baby platies? It might attack them if the babies are too small i think...

 

[FightClubChick]

Its a whisper 5 that came with the tank and im pretty sure its not fin rot. its looks bitten. ive seen them swim by the filter before and it not bother them so i dont think its too strong. I was afriad it might be when i first got them so i always watched to see if it affected them, and it never has. Plus, they are much larger now then they were a month ago.
also hes swimming a lot more now that hes seperated and not hiding as much.