"I add a bag or two of crab sand (calcium carbonate) on top of a primary substrate of regular sandbox sand when I clean out the crab tank (every six to eight weeks, since we have only a small pair of hermits). Crab sand gets incidentally eaten, and adds much needed calcium to the normal hermit crab diet. Crabs that have molted in the new environment have been redder in color and much healthier looking than before. Since I haven't changed much else about the habitat, I attribute the positive change to the addition of the crab sand.
Always remember -- hermit crabs have modified gills, and need a moist environment. Don't just keep them in a reptile tank with the cover off unless you live in a very humid, consistently warm environment -- they'll suffer and eventually die of suffocation. We've added a humidity and temperature gauge to the inside of the tank, and we keep the glass cover on the tank and a clean sponge in the water dish almost all the time to keep the humidity level where it should be (70% to 80% humidity and 70 to 80 degrees Fahrenheit is ideal). I also have a heating pad on a timer so that it creates a temperature gradient at night, when it can get a bit cooler."