I have check my tank with a laser temp gauge, on the ground I have temp varying between 78 to 85 but on the second level my temp was between 85 to 90 but crab seems to like the second floor.
If they all like, go to the top more than the bottom, maybe they really like it?the sand in the beach is easily 85 to 90 degrees, so i dont think its bad, check with the others though.