Sure, they require brackish water and some way to get dry/out of the water. If you only filled it 2/3 of the way up and left a rock or something sticking out, it would probably work out.
I wouldn't be so hasty b rob. Crabs are known to be able to catch small fish and eat them. Especially so in such a small tank. There is always a possibilty. A free meal is a free meal.:dance2: