I've managed to catch my loaches before, but I have never, ever been able to snag my pleco. I've had it since it was 3 inches long and it's never been out of its 10-gallon tank, which was originally the quarantine tank when it first came from the store. It's about six or seven inches long now.
Like Gunnie said, get everything out of there and lower the water level. Then prop up a large net in the tank. Your fish should soon get used to the net. After a few hours or a day, stick another net in and see if you can chase him into the net. Otherwise try catching him with your bare hands. Be careful, since many catfish have spines that can hurt you or get caught in a net.