For an 18,000-gallon above ground pool, a 38 GPM flow rate is necessary to keep the water from becoming stagnant. We recommend stepping up to the Black+Decker 1.5 HP Variable Speed pump. By pairing this with the 1200 Series Cartridge Filter, you get a 'Pro-Grade' filtration system. The larger 1200 Series tank provides significantly more surface area than standard entry-level filters, meaning you won't have to clean the cartridge as often, even during heavy swimming months or high-pollen seasons.