When catching fish, stop feeding them. After a few days, just put the tastiest food in the trap - mysis work well. Once they get hungry enough, one or two will go in there. The others soon will follow. Mine has a long piece of fishing line that I can hide off to the side and then let it go. I have always gotten my fish this way, but a few stubborn ones can take a week. The brave ones get fed when they go in there, but it seems that the one that you want to catch is never the first one to go in.
The puffer probably sleeps in the rockwork, but it if sleeps in the open, you can get them with a net first thing in our morning when the lights are out. This rarely works for me except for clowns that are in nems and other stupid fish that sleep in the front corners.... but worth a try.
All traps are different, but you have go have a way to keep the food in the trap when the flow is on.
