Fishtraps are useless for catching holding fish as they chill out in the back and you cant lure them out with food...
I use 2 nets the biggest ones I can find that fit in the tank (for my big tank I have a fishing net from the warehouse) move slowly and try and move them gently into the big net.. Its when you start moving frantically that things go badly.. Usually I have to take all the rocks out to catch a fish, I leave a few in for the holding fish to hide in so I can hopefully flush them out and into a net.. Its all hit and miss and takes time, but one thing you can guarantee is you will never catch the one fish you want...