When I brought home the tank I bought through Craig's List, it already had sand in the bottom of the tank. As I set the tank back up, I just put the rock back on top of the sand. I read that it is better to set the rock on the glass of the tank and then put the sand in. Since I need to clean some dead algae off of some of the rock, would it be OK to move the sand around and replace the rock on the glass, then settle the sand back in place. Of course, any of this has to be done around the urchin, since he and my hands don't get along.