I got this suggestion from this forum and it worked for me. Get a liter bottle and ccut off the top third approximately. Then take the piece you cut off and turn it around and place into the other end, Make sure that the original hole where the cap typically goes is big enough for your fish to swim into. If it is not cut just enough so he can fit. (Don't make it too big) I then had a piece of fishing line tied through a hole so I can pull it up if I need to. After letting it float around for a few days so they would get use to it I placed food in there. The fish I needed to go in finally went in to get the food! Once he realized he was in there he couldn't figure out how to swim through the small opening fast enough and I had pulled the stringand had a net ready to cover the opening. Took a few days but I got him without tearing down my rocks in a 300 gallon. Good luck! Remember the opening must be just big enough that his curiosity and hunger gets him to go in.