One of the most important parts of your "crabitat" is the sand. A hermit crab depends on sand for cover during the molting process. A sand substrate keeps Hermie safe from predators when he's most vulnerable, and it prevents him from dehydrating while he's without his shell.
Sand Like Home
Hermit crabs hatch when their eggs are flung out to sea by mom. Some of them wind up in the pet trade, because these creatures have yet to be successfully bred in captivity for retail. It's important because it influences the habitat you provide for your hermit crabs. The best captive habitats mimic their natural habitat, which usually means using a substrate that's similar to a combination of soil and sand. A mixture of coconut fiber and cleaned coarse sand is the ideal substrate for Hermie's crabitat. The coconut fiber helps keep the sand moist; the sand packs well. Combined, they provide a hospitable environment for Hermie when he molts. Some hermit crabs go underground for up to three months, so you need a low-maintenance substrate that allows him to be undisturbed during this critical process.
Good Sand Choices
If you want to stick with sand, start with basic, all-purpose sand sold in your local hardware store. This light-colored sand tends to pack well, is fairly coarse and retains moisture, filling all of Hermie's burrowing needs. If you're lucky enough to live near the beach, you can try using beach sand, provided it's not too fine. Beach sand may have shells and other beach debris in the mix, so go through and pick out any sharp pieces that could poke or injure Hermie in his naked state when he molts.
If you want to try to duplicate Hermie's original environment, it's pretty easy to mix the combination substrate of coarse sand and coconut fiber. The coconut fiber comes in a hard brick form you'll soak in water in a large container. After it has soaked, break it apart with your hands and mix a measured amount with the sand until it's thoroughly combined. You'll use about 1 part fiber to 5 parts sand; aim for a consistency that's suitable for building a sand castle. The substrate should be easy to dig in, not too dry or too drippy. Premixed substrates are commercially available if you want to avoid the time, trouble and guesswork of home preparation.
Sands to Avoid
Play sand isn't an ideal choice for your crabitat. Unlike all-purpose sand, it doesn't retain moisture well, is finer and occasionally has metal filings that could harm Hermie. If you choose play sand anyway, make sure it doesn't have any additives, such as antibacterial agents, that could be harmful to your crab. Ironically, hermit crab enthusiasts often shy away from packaged hermit crab sand. Not only is it more expensive than all-purpose sand, but it can be quite dry and stick to the crab's damp abdomen.
Check Your Sand
If you smell something "off" when you open a bag of sand, don't use it. Before you mix your substrate, take a moment to examine the sand and run it through your fingers, checking for bugs or other debris. Individual sand bags can be punctured or compromised, with insects burrowing inside or mildew-causing moisture getting trapped in the bag.