The short answer is yes, you can put rinsed sand in it. The rock doesn't need to be on the bottom, the issue is STABILITY. Keep in mind that critters will dig underneath the rock and make it unstable. My rock is on top of the sand, just buried really deep into it. I try to move the rock around just a bit occasionally to make sure it's seated well when something has undermined it. If the rock is on the bottom I would suggest something like starboard underneath it to pad it from the glass.