It's pretty easy to descend fish with a homemade descending device and a heavy sinker. Here's the one I made for when I fish deeper than the allowable depth for rockfish. A 1+ lb sinker will have no problem taking small to medium fish back down. Bigger fish would probably require 2+ lb sinkers.
With that said, I think you can tell some of the fish aren't going to make it no matter how gently you handle them or how quickly you send them back down.