My blue damsel has recently started clearing a large space on the bottom of the tank. He has cleared away all the sand by fanning it, & the rubble that's too heavy to be moved that way gets carried away in his mouth & deposited elsewhere. Now there's a baseball-sized spot where the acrylic is completely clear. He even drags the starfish away if he gets too close. Any ideas about what he's doing, or why? Is this normal? Thanks.