Hermit crabs require a fine sand-like or soil-like substrate at the bottom of their tank so they can easily bury themselves when they are ready to molt. The most popular aquarium substrate for a hermit crab habitat is sand. Other substrate options include finely ground fiber crushed coral. Avoid gravel or wood shavings, as these do not work well as hermit crab bedding.
If you aren’t sure that you’ve chosen the right terrarium sand, observe your hermit crabs. If you notice them digging themselves into the sand, then you’ve picked a good aquarium sand substrate.