Do you know where he sleeps? I've also had success after the lights go off, waiting about an hour and then turning the lights on full and using a net. Many fish are stunned by the light and you might be able to net him. Also if he's in a cave of a rock you can fill a container with tank water, let him go into the rock, then pull the rock and put it in the container. Give it some time and he should come out, pull the rock and you have an easy catch in the container.