Green algae in swimming pool — Delft, Zuid-Holland

Green, slimy water and algae growth on pool walls or floor indicate that the sanitizer level has dropped too low, the filtration system isn't running enough hours, or the pH is out of range. Left untreated, algae can clog the filter, stain the plaster, and make the pool unsafe to swim in. A pool technician can test the water chemistry, perform a shock treatment, brush and vacuum the surfaces, clean or backwash the filter, and adjust the chemical balance to prevent recurrence.