I opened my 38,000 IG vinyl pool this year to find that the 30-40ppm CYA that was present at the close in the fall is now at 0?? This is the second season that this has happened. I tested with the black dot test and then had two different pool stores check it. All came up 0. I do not drain any water off to close and had to only add about 1" of water this year when I opened it.