It's been a few days since I tossed some sand in the bottom of the little dudes tank. I've actually noticed a huge difference in his demeanor. He's considerably more confident as he moves around his tank. He's even starting to buzz up to the front of the glass when I come into the room. In my opinion a bare bottom tank would be fine for some species or during a quarantine period, after that it would certainly be beneficial to the animal to have some type of substrate.