I am thinking about getting some hermit crabs but I cannot decide which substrate to use. I want to create a very natural looking environment for them because it will be used in a high school classroom. I would really like to use sand such as calci-sand because I think it looks more naturalistic but I want to use what is best for the crabs. I cannot decide whether to use sand or the coconut fiber bricks. I know that the bricks are good for humidity but what about calci-sand. Does it hold moisture fairly well?