These little guys are sitting in cups and small spaces often for quite awhile and lose muscle strength. And sort of lived alone. They all have their own personalities. Take that into consideration.
But also how are your water parameters? Did you acclimate him?
Is the tank run through and fully cycled, like over a 2 month period? Info about aquarium keeping / cycling, if interested is a link in my signature for lots of good tid bits and helpful /useful information.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves