Phosphate is the major cause of algae in swimming pools. Phosphate and nutrients are required by all living organisms — including algae — to survive and flourish. Phosphate is introduced to pool water from a variety of sources, including rain, runoff from lawns and gardens, fertilizers, birdseed, plant material, dust, suntan oil, and leaves. In time, all of these sources will cause a buildup and increase the concentration of phosphate in the water and thus promote progressive algae growth. There are phosphate removers available to reduce the level of phosphate in your swimming pool water below 100 ppm (ideally, you want it as close to a zero reading as possible).