Having crayfish really depends on the cray itself. When I bought mine, I bought her with the thought that my only other fish in the tank, my Phantom Tetra, was going to be food. Well, 6 months later and I have added 3 Tiger Barbs to the mix, and I am getting another tank ready for all of them to have more room. Also, I'll be adding another crayfish as well, plus some other fish.
Crayfish are funny, some don't mind company while others....not so much. There are various web sites out there with cray info, if you are interested. I can post some here, or you can do a google search.
I'll forewarn you though, once you get the first cray....you'll wonder what you were doing before, as their antics can be quite funny.
