The Gravel-20 is a double scrubbed, filter media gravel. This is a 50 lb bag of high purity, high quality gravel. Scrubbed to remove organic contaminants, this gravel makes for ideal filtration. Gravel-20 is used in commercial tank re-bedding as a top layer of gravel. A unique property of Gravel-20 is that it is naturally clean making it a key component for large aquariums.