Swimming Pool Alum (Aluminum Sulfate) is a water clarifier used to remove cloudiness caused by fine suspended particles that standard filters cannot catch. When added to pool water, alum causes microscopic debris to clump together into heavier particles, which settle to the bottom of the pool. This process, called flocculation, significantly improves water clarity. After allowing the floc to settle for several hours (usually overnight), the debris is removed manually by vacuuming the pool floor. During treatment, the filtration system should be turned off, and the water’s pH should be maintained around 7.0 for best results. Alum is especially useful after heavy rainfall, pool parties, or algae treatments when water appears dull or hazy.