Great fish, and amazing pics. I wish I could get a pic of one of my 2 BGK... However, I have seen those tanks in the flesh and I don't think they will happily house a full grown bgk.
Unfortunately those crabs naturally inhabit brackish waters and certainly thrive in water with marine salt added. Together with the fact that crabs will eventually catch any fish they can they are not really a must for every tank. I would be surprised if he survives once the bgk gets to neare adult size.
The problem with crabs with fish is that the fish that are usually large enough to be safe from predation from the crab will look upon the crab as food.