Caribsea Seaflor Aruba Puka Shell Fine 18 lb. bag. This product is the perfect shell substrate. It has buffering capabilities superior to oyster shell, coquina shell, calcite, dolomite, and most varieties of beach collected shell because of its aragonite component. Clean and pure; no dangerous impurities or harmful organisms to endanger your aquarium. Available in 18 and 44 pound bags.