I forgot to mention that I also have 3 Stone Crabs in which one is still in the tank. The last one I got out was bigger than a quarter and between the Mantis & Stone Crab they killed all the Hermit Crabs and all I've are Snails. Can't put any Hermit in till I get the last Stone out.
EyeOnAll,
They usually come out during feeding, so watch closely. If you have a Mantis it usually go from rock to rock even with the lights on.