I've been having a hard time keeping the chlorine levels in range in our spa. Never thought it would be so difficult but the main problem seems to be widely variable levels of chlorine in bleach. They're all over the place and the label and the manufacturer are no help. I never realized it was such a rapidly degrading product. It was helpful to come across the formula on this site for testing bleach without any other variables by diluting in water and using the FAS-DPD test. Some folks said that the test could have big errors but I tested a new bottle of Walmart Pool Essentials 10% and got a reading of exactly 10%. Clorox 6% came in at 3% and I was surprised to find Cloralen Mexican bleach which doesn't list strength came in at 4%. No wonder I'm not getting consistent results with Pool Math.