Many years ago, I had two pearly jawfish. Pick forget how much depth of substrate I had. It was probably about 3 or 4 inches. When they construct their burrows, they collect pebbles and artfully arrange them around the burrow. When one fish was feeding, the other would take he opportunity to steal a pebble. Then the first fish would try to get it back. They squabbled endlessly over their treasures but never stemmed to harm each other. It was very entertaining. I would recommend that you provide enough coarse bubble and shell fragments for your jawfish to have their trophy collection.