Substrate is the substance placed in the bottom of your hermit crab tank, giving them a substance to dig in. Be careful when choosing substrate and make sure you buy enough substrate to fill the tank so that your largest hermit crab is capable of fully burying itself under the substrate. Pesonally I prefer Eco Earth substrate, but that is my personal opinion. Gravel is NOT a good substrate choice, as it may get stuck in the hermit crabs' shell, causing injuries or making the shell uncomfortable for the hermit crab and they cannot dig in it. Sand is kind of medium, they can dig in it and calci-sand can be eaten by the hermit crabs to provide them with adiquate mineral amounts. Eco Earth substrate is the best substrate I have come across so far, it is made of shredded coconut husk and not only can they eat it, but they can dig in it as well and it will not get stuck in their shells like gravel and (Sometimes) sand. Some coconut husk substrates claim to break down waste matter, though I am still researching on this and will have more information later. Make sure to keep your substrate moist and at sand-castle-building consistancy as well. Sand and Eco Earth substrates can be mixed together to make a delux-of sorts-type of substrate