Detailed Product Description Gravel filter, also known as cobblestone filter, She produced 2,500 years ago the Earth's crustal movement pebbles, gravel among the After screening more than ten working procedure is made She is a pure natural filter material, She has a strong adsorption, precipitation of, resistance to stress, Specification :2-4mm; 4-8mm; 8-16mm; 16-25mm; 25-35mm; 35-45mm