Mix different textures of sand together to form the substrate. Fiddler crabs tend to dig burrows in sand, so the substrate should be sturdy and hold together. The best way to achieve this effect is to purchase sands of 2–3 different consistencies (fine, coarse, very coarse) and blend them together. Pile the substrate at least 8 in (20 cm) deep so the crabs can dig and burrow.
Purchase sand of 2-3 different consistencies at a home-improvement store, hardware store, or home-supply store. Or, if you live near the beach, take a walk and collect coarse and fine sands yourself.
To ensure that the substrate doesn’t dissolve into the water, you can line the bottom of the tank with a single layer of pebbles or pea gravel.