You'll need about 1.5 pounds of aquarium gravel for every gallon of your tank to create a 2-3 inch layer of substrate. Rinse the gravel before adding it to the aquarium. You can choose, instead, to use 2-3 inches of fine reptile bark bedding to help maintain humidity.
You can add artificial plants to your crabitat for decoration — Hermit Crabs may eat live plants — as well as climbing areas like ornaments and rocks and hiding places designed for aquariums.
Check the humidity and temperature levels daily to make sure your Hermit Crabs are comfortable, and clean the habitat decor and gravel on a monthly basis. You'll also want to replace the bark bedding every month to maintain a clean and healthy living environment for your Hermit Crabs.