Some fish wander in right away for the bait, clownfish come to mind, the little pigs. Tangs tend to be more skittish. If you spook them, they may not come back for a very long time. You might want to let them swim in and out of there to feed for a couple of days before you make your attempt. Make sure the door will close properly (the line doesn't get hung up on the canopy, etc) beforehand too.
Attaching the bait to a small rock with a rubberband or to the bottom of the trap with a dab of putty or silicone will ensure they have to stay in there to feed rather than a grab and dash.
When you decide it's time to try, get yourself a comfy seat and a cold beer; hopefully you won't have to wait very long.
i second that one,
patience is the key. lull them into a false sense of security by letting them
get away with it for a while before attempting to trigger the door.
i had to round up a yellow tang a while back, took six days of persistant staring at the tank. several failed attempts later, i finally got him.
If you can remove the rock from one end of the tank then you could chase the fish (or entice it with food) to that end. Then place a barrier such as a piece of acrylic between the fish and the rocks. That should make it a lot easier to catch.
A trick that has worked for me, put the net (I know they are bad) in the tank and leave it hanging on the tank wall. Dont feed for two days, feed and grab, whaa-laa a fish in a net. You really only have one shot with this method so use a big net and make it count. Hope that helps someone. No real input on the fish trap. I have used a soda pop bottle on its side, but just to catch a Diadema dottyback. He was a mean little bugger.