Having the right kind of substrate for your hermit crabs is an important step in proper care. Here is the method I use in my own crabitat. What kind and how much do I use? Play sand mixed with 100% coconut fiber or coir. Some mix 5 parts sand to 1 part coconut fiber (5 cups sand to 1 cup coconut fiber). Others add 1 brick to every 50lb. bag of sand, while others add even less. The argument is hermit crabs do not live on 100% sand or forest floor/soil in the wild. Instead they tend to be found on a mixture of both mediums. I've tried 100% play sand and 100% coconut fiber in my crabitat but have had the best results from combining both. The humidity is very stable, my crabs tunnels do not collapse and molts have been even more successful. Play sand is cheap and could be found easily. Home Depot, Lowes and Walmart just to name a few. Coconut fiber can be found at your local pet store or purchased on line. ZooMed brand makes a good one called EcoEarth. Follow the directions on the package on how to expand it. You may need to wring out excess water from the coconut fiber. How deep should it be? I provide 8 inches of substrate sometimes more. Your substrate should be at least 3 times deeper than your largest crab. In other words, if you have a one inch tall hermit crab your substrate should be four inches deep. The deeper the better. Don't forget that your substrate may settle over time, so add extra! How moist should it be? The consistency of your substrate should be like trying to build a sand castle. As long as it holds it's shape when you poke a hole or dig in it. Careful you don't add too much water. If you do just add more sand. What can I add to my substrate? Mixing in some crushed oyster shell will provide your hermit crabs with a calcium source. Also, when they are buried for a molt they can eat it for a calcium boost. Marine grade sea salt can also be added. I expand my coconut fiber using sea salt. It helps keep mold at bay and the crabs seem to like it. What should I NOT use? Calcium sand will stick to your hermit crab and everything else possibly damaging their soft abdomen or gills. It will dry to a rock hard consistency when wet leaving your hermit crabs no way of burrowing. Gravel also does not allow for burrowing and should not be used. Cypress bark and bark chips are also not recommended.