I got this platy when I first got my tank. He used to swim around a lot, at the top, in the middle, and low close to the substrate, fins open, chasing the two female platies. He still feeds and if I wave my finger in front of him, he'll open his fins, but otherwise he just kinda hangs there, moving back and forth at the top, looking like the pictures below.
The original females had given birth to other females, and he would chase those around, also. He started acting like this once the last of the original female was on her last legs. He's been like this for over a month now. I've since removed all the females and they're in my brother's 150 gallon. The tank he is in is a 20 long heavily planted community tank with weekly 2.5 gallon water changes. Water parameters normal. All other fish are healthy and active.
Other fish include five Neons, five Harlequin Rasbora, a male betta, a honey dwarf gourami, two kuhli, three false julii coris, a couple dwarf frogs, a 2 inch placo, lots of red cherry and crystal shrimp, and three amano shrimp. Filtration is a 66 gallon EHEIM canister, with purigen and chemipure, eco complete substrate, a log, and a rock.
If there's any other information needed please comment I'll be glad to add it.