CaribSea aragonite is purposely engineered and 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.1-1.7 mm diameter grain size. A revolution in reef keeping! This is the one 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. * Heavy Item - please select Heavy Item in shipping options during checkout to obtain accurate shipping charge.