Gravel filter is a different filter in the filter which is necessary cushion, our factory produces natural gravel (was elliptical-shaped variegated) and mechanical processing pebbles (similar to spherical, pink white) Material nature is very hard, It can also anti - pressure and wear-resistant, is the best cushion filter material, packing density 1.8 tons/ m³, Mohs Strength is 7.5, hydrochloric acid soluble rate is Less than or equal to 3.5 .