Mantis Substrate

It depends on the species that you have. A typical smasher such as O. scyllarus will do fine in either, but a burrowing spearer such as Lysiosquillina maculata does better with sand.

Most of my animals (O. scyllarus, O. havanensis, G. chiragra, G. platysoma) are kept in aquaria with crushed coral and shell with larger pieces scattered in. I give the animals lengths of pvc for cavities and they modify them with the bits and pieces of coral.