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I think you missed the parts where the holes would be above the sand line, which means the sand would never touch any holes and that flow would be directed not just at the surface of the sand, but at the edge where it meets the container, driving flow deep into the sand bed and out the other side as the flow hugs the curvature of the container.The sand inside the plastic rock would fall into the holes, clogging them, and in order for the rock to be feasibly printed, the holes would be too few to let flow through. (Good) live rock doesn't get clogged because it is filled with pores and there is nothing inside the rock to clog the pores. Having the sand sit in a basin within the plastic rock would not give you enough surface area for ample nitrification. Remember that the pores in live rock are much smaller than the space between sand particles.