I have a trap that I use,that is made out of acylic.Its basically a round tube,with a trap door that you slide shut,when the fish is in the trap.Its not the quickest way sometimes,but it hasnt failed me yet.
Basically what I do first is put the trap in,and I always let it in the tank 24/7 when I need to catch a fish.Now,once you have the trap in,ALL food you normally feed needs to be put in the trap only.This gets them used to having to go into the trap to get food,and basically if they dont go into the trap to get food,they simply dont eat.Some fish take longer than others to go in,and be able to trap them.I've caught fish within 20 seconds of me putting the trap in,and others have taken a couple days.The key is being patient.
Mike