poke holes in periphery of bag's opening. CLEAR Zip locks work well as opening has rigid surface that can be folded over, and will stay open. Belt loop fishing line around the opening. Tie a slip knot in the line. Test it. Give it a quick tug and make sure it sucks shut quickly.
Turning off flow will help you. Place a small piece of LR in there or anchor it in an opening in rocks. Put favorite food of perp in bag. Sit still with string in hand. Be prepared to watch every fish you are not trying to catch swim in and out of bag. Sooner or later fish you want WILL go in for food. When he does, yank the line. Presto, fish in a bag.
I've had to do this twice. I tried for literally WEEKS to catch a pygmy angel. Tried this method. Had em in the sump within 10 minutes, no exaggeration. Did the same with my sailfin, took even less time.
Incidentally, both fish were found innocent of all charges and returned promptly to the general population display tank.