Sand filter
Also called: sand filter tank, high-rate sand filter.
A sand filter pushes pool water down through a bed of media, #20 silica sand or recycled glass, and dirt lodges in the gaps between the grains. When the bed fills and the pressure climbs, you backwash: reverse the flow and flush the trapped dirt out to waste.
Why it matters
A sand filter is the set-and-forget choice: it shrugs off a heavy debris load and is the cheapest type to run, and what you pay instead is water, every backwash. Size the pump to it. Sand filters work best at roughly 15–20 gallons per minute per square foot and start to channel above about 25, so an oversized pump filters worse. Refill with the grade the manual names: Hayward and Pentair both specify No. 20 silica at .45–.55 mm, and finer sand passes straight through the laterals into the pool. Replace the sand on symptoms, not on a birthday. On an above-ground pool, sand wins above about 12,000 gallons.
In numbers
Every sand filter has a maximum flow the pump must never overrun: a 16-inch sand tank maxes around 30 GPM, an 18-inch around 35 and a 24-inch around 62.