Filter sand

Filter sand is not play sand. It is a sized, sharp-edged grade made for filters. Over time, the grains wear smooth and clump with oils and minerals, and the filter stops catching fine particles.

Many makers suggest replacing it every few years, so check your manual. Signs include cloudy water after a backwash, fast pressure rise, and sand returning into the pool. Changing sand takes work, and a damaged lateral under the sand needs a fix as well. A pro can handle it. See sand filter.