I have only had the jawfish for a few weeks, so I'm no expert, but I made a small pile of crushed oyster shell in a front corner of the tank a day after adding the fish and he moved in right away. He seems to have no problem at all moving around or digging in it. In fact he has moved some pretty big chunks of rock rubble over to his home. They seem to like to have a piece of dead coral branch or something similar over their den and a few pieces of shell to use as a door.