Thanks for your replies, much appreciated.
There's still alot i need to know, i will be keeping the crab with angels, clown loach's, neons, zebra danios, dwarf gouramis, candy striped pleco etc etc.
What i may consider doing is getting a divider and split my tank up so that the crab or crabs have their own space.
The things that worry me are how can i have the water brackish when there are going to be alot of other types of fish in the tank that dont like brackish water?
I may also not fill the tank right up so that the crab has access to dry land, I just want to get everything sorted out and planned before i got one or two of these crabs.