you'd be surprised what a crayfish can catch. i had one for about 3 years in a 20g, and by the end, he was kept alone. he caught a half dozen zebra danios during a few months in a setup with them, and you're hard pressed to find faster fish for a 20g-sized tank! the only fish that survived with him long-term was a very big, very mean male blue gourami, but i wouldn't even recommend keeping something like that with a crayfish. best to keep them in a species tank, maybe with some inexpensive fish like guppies that breed a lot, so the cray keeps the pop in control.
although, i'd bet he wouldn't eat fish you *want* him to eat.