you can also go to a pet store and buy one of the clear critter cages with the colored tops. They have a door hatch in the top and work great as a trap.
just go to youtube and search for a fish trap make one out of a small water bottle or 2ltr bottle depending on fish size. They work awsome. leave it in tank a few days without feeding the tank then put food in it. And hope he is the first one in