The ideal substrate for all saltwater fish and invertebrate tanks, Reef Base consists of naturally occurring reef sand spheres mixed with aragonite coral chips, and comes prewashed and impurity-free. The spheres, highly porous calcareous shells of simple protozoa (foraminefers), provide a large surface area for bacteria colonization and serve as an excellent media for both aerobic (nitrifying) and anaerobic (denitrifying) biological filtration. While the high buffering capacity of natural aragonite, the most dissolvable form of calcium carbonate, helps to maintain natural, stable levels of pH in the aquarium.