What should I do with this platy?

[af2018]

I always get a head count of my fish and this little girl or guy was deep in hiding, interested in food and would pop its head out but pop right back into hiding. No visible wounds or tears. The fish looks just fine. It drives me nuts not being able to get my counts in a timely manner plus it seemed miserable so I popped it into my 2.5 gallon. As you all should know I don’t see that tank as a permanent solution, I refuse to set up another tank. I’m trying to figure out what I should do with it. It’s perfectly healthy and normal by itself. Any ideas why this one fish could be so unhappy in an understocked 55 and then tickled pink by itself in a 2.5?
Just to add. It’s been eating by itself too. Like I see nothing wrong with it. And it’s completely normal now I’m stumped.

 

[Andy Sager]

I get the feeling that this is an immature male that is getting harassed by a more dominant male or larger females. ( If you look at the anal fin in pic 6952, there appears to be an extension to it which would be an evolving gonopodium. ) The key is that alone, it's doing fine which means it's something in the main tank. That could be anything from fish to decorations ( or lack there of). If you have other fish in the tank that are doing fine, it's not the decorations, it's the other fish that are making this one feel insecure.
Your choices are a new tank or rehoming this fish to a tank with no other male platies. The up side of keeping this fish is that you know it's a younger fish so you should have it longer than a more mature fish and if something were to happen to the male in the main tank, you'd already have a replacement for him.