Most Bettas are raised in jars, for aggression reasons, but also so that swimming won't cause fin tears and affect their value to buyers. When they first go into the tank, they have the muscle tone you'd have if you were raised in a jar! It can take them weeks to get to strength - your fish may never have had the chance to swim since he was sexable, until when he was put in your tank.
Add to that his fins, which are not natural, but are bred for sales. Yours isn't too handicapped, and should recover enough to get around.
Problem two is that danios are racers who need a current. Platys come from rivers as well as stiller waters, and have a body shape to enjoy a good current. Your Betta looks like it's fallen into a creek in a wedding dress. The drag from the breeder selected finnage is considerable.
So you either cheat the danios and platys by reducing the reasonable flow, as the Betta will be exhausted. You set the filter flow differently, so he has some quiet protected surface and rest areas to go to. Or you get him a quiet, heated and filtered tank.
