Our famous Reef Saver Rock cut to offer a smooth, flat single surface. We recommend placing this rock under any rock work at the boundary layer between the glass and rock display. This design allows the second layer of your scape to safely "float" above this bottom layer, promoting healthy flow around the rock work and over the substrate. Foundation Rock can be siliconed to the glass if desired. Another aquascape option is placing the rock flat side up for a shelf look. Smooth Cut Bottom Stable and Architecturally Sound Our MarcoRocks is the finest all natural aquarium rock available to the aquarium enthusiast today. It is a once live natural reef rock. All natural calcium carbonate rock. Never any chemicals, additives. Often, you can spot fossilized ancient corals within the rocks. Highly Porous and Beautifully Shaped