Caribsea Reef Rock Sand, 5 to 6-Pound At an unprecedented 50% void space (macro and micro), you will be hard pressed to find a durable natural rock that has this magnitude of porosity Natural weathering processes form the fantastic shapes of South Seas Base Rock by the erosion and pitting of limestone over the course of eons T is clean and safe for marine, reef and hard water freshwater aquariums such as African cichlids