Not all sand is created equally. Some sand is silica-based, while others are made of calcium carbonate. Silica sands typically have a smaller grain and do not offer any buffering. Black sand is a very popular silica sand, as is the superfine playground sand that made it into many aquariums during the ’90s. Calcium-based sand can be of multiple grain sizes and is more natural looking for most reef aquariums. Most reef keepers choose the more natural looking calcium carbonate sand not only for its looks, but also because it helps buffer your tank’s calcium and alkalinity levels.