It would also be a good idea to ask brackish water aquarium questions in the brackish water section. That's where us brackish water people tend to hang out!
And yes, as boboboy says, there are crayfish in all kinds of waters. There certainly are a few species found in slightly brackish water, buy by and large it is crabs and shrimps that occupy the crayfish ecological niche in estuaries and mangroves. The salinity tolerance of the common European and American species of freshwater crayfish has been quite widely studied by animal physiologists, and is not very great, so unless you know better, leave them out of the brackish tank. If you have SG 1.010 or more, consider the blue-leg hermit crab, Clibanarius tricolor. This animal is usually sold as a reef tank invertebrate but is very euryhaline and does well in brackish water aquaria. Moreover, it is harmless to other animals, unlike crayfish, which can be distinctly predatory.
Cheers, Neale
