Premium Aragonite Reef Substrate #4 is comprised of gravel that comes straight from the ocean floor, it is pre-washed and heat sterilized. Aragonite found in the substrate helps reduce nitrates, maintain pH and increase carbonate hardness. Ideal for reef system tank bottoms, culturing live sand, denitrifying anaerobic beds, and extremely high surface area filter media. Premium Aragonite Reef Substrate #4 features gravel with grain size ranging from 1.7 to 2.5 mm.