We treat strictly with chlorine. We use an algaecide 60 once a week and a few ounces of metal hold weekly as we do keep the water level up with well water. Two years ago we put a new liner in and the pool was filled with pool water. Since then, most of the pool water is still in and as I stated earlier, we just increase the water level with well water. Our well water has been tested and is free of any phosphates although I haven't got a full water check yet this year. But this mold was present before we even put any additional water in this year. When we closed last October the water was crystal clear and tested perfect with all the chemicals at the proper levels.