I recently purchased three Neon Gobies for my 90g reef aquarium and after a four days in the aquarium they have completely disappeared! Initially they were schooling in the water column, eating fine, and gradually moving more and more toward the live rock. I went out for an hour today and when I came back they had disappeared! I checked my overflow, sump, etc and they are nowhere to be found. Is it possible they have burrowed into the sand or live rock?