red claws are fun and small. I have 2 in a 20 gallon tank that is 2/3 full, with 7 cherry barbs and 3 ottos. the crabs absolutely stalk the fish, but the fish are too fast for the crabs. the crabs even let the fish clean their caves, etc now, it seems like they've given up on catching them.
the two I have have very different habits. one spends most of his time out of the water, sitting at the very top of the log that acts as water fall/return. he seems to like eating moss and bits of wood as much as worms.
the other crab hides inside the log, 1 inch below the water surface. he comes out of the water often, but has chosen the log hole as a home. he guards it, to a degree, but not too strongly. I have seen this one head to the gravel floor to grab up worms when the other fish are fed, and never seen it eat moss. he seems to pick at the floor of the log hole often.
my tank has a tall 4 layer stone pile that creates several caves and crevises, as well as the big log and plants.. I like the set up a lot. knowing now how much they like to climb, I wouldn't suggest a beach set up as much as an estuary like environment with rocks and logs.