I'm sure there are some ways to catch it without tearing apart your aquarium. You could place a short piece of PVC pipe (1-2" wide) in the tank and hope that your crayfish hides in it for easy removal.
Hmm, my bad. I forgot you had a crawfish in there. Um, have you checked out some of the traps they have for SW pests like mantis shrimp and stuff? Some of those I have seen would work for a crawfish, I believe. Check them out at www.drfosterandsmith.com
Nice Ideas thanks. I read up on the Mandarin newts and I like that idea but it looks like they are not very easy to get your hands on them. Since I haven't been looking for newts in local stores I'm not sure how easy they would be to find. I know you can find fire belly newts quite often and they are really active in the water too.