Again when u have a number of piranhas(wouldn't call it a school)of carnivores like ur rbp's, usually its just a matter of time until the other tankmates die. The thing about the crayfish is its night active and the rbp's are active during the day, so if you can make a cave for the crayfish to be safe in, then most probably he will survive in your tank. As long as you feed your rbp's well on a daily basis and have a lot of space in your tank for your fish. But with rbp's in your tank theres never a gaurantee. GL.
