Even with proper filtration, pools naturally collect debris and contaminants due to: + Environmental Debris Leaves, grass, dust, and pollen are blown into the pool by wind. Nearby trees and landscaping can increase buildup. + Bacteria & Algae Growth Warm, still water can become a breeding ground for harmful microorganisms and algae if not treated properly. + Swimmer Contaminants Sunscreen, sweat, body oils, and even small amounts of dirt are introduced every time someone swims. + Weather Changes Rain can alter pool chemistry, dilute chlorine levels, and introduce organic material.