Hi daz,
I was going to setup crab tank at sometime, so i did look into it, if you do a search on the FK mayb it will come up with some topics.
Some things I did find out tho that yes most crabs that are sold as freshwater are in fact usually better suited to brackish, if you think about it that is just common sense because as we all know many crabs live in harbour mouths and the areas where brackish water is present.
They are very messy apparantly also much as goldfish are but of course they can poo on land which is something that needs cleaning as well as the water.
They need a dry area such as a beach or rocky outcrop that sits out of the water.
The tanks also need to be fully sealed, because the crabs are very good escape artists and will make every attempt to climb out and go on the run.