I have noticed in the past couple of days that my maroon clown is digging a pit in the sand bed. It will removes the sand pieces out with its mouth and sometimes it gets really close and goes to shaking its body to carve out the gravel. I only have the one maroon and a banggai cardinal in the tank. Is this some sort of nest building? The maroon and the banggai stay close to each other around the depression in the sand.