A revolution in reef keeping! This is the substrate you've been reading about in all the hobbyist publications. This grade of aragonite is specifically engineered for plenum-type nitrate reducing beds. The pore water space created by precision grading also allows maximum ph support and dissolution of calcium carbonate. Free of impurities such as ash, metals, pesticides, and silica which allows you to create a safe and beautiful environment for your fish and invertebrates. The 15 pound bag of CaribSea Seaflor Special Grade Reef Sand has a typical grain size between 1.0 mm and 2.0 mm and has an average density of 85 pounds per cubic foot.