I added a diamond goby to my 20h 6 days ago. He started sifting sand a but then the next day I couldn't find him. I saw where he dug a lil hole under a rock. So I though he was makin his burrow. Well I decided I wanted to trade him back to the lfs cuz I never see him. I did a water change and re arranged my rock, I look underneath the rock and he wasn't there. Dug in the sand, could find him. Is it normal for these to completley bury themselves?