I did it again and this time it was almost to the twenty. About half a finger width below the 20 mark. This is when it was in the shadow of my body. When it was in bright sun, I could still see the dot. I don't know if that matters.Are you sure you added the pool water to the diluted test? Or the test solution to the pool water/tap water mix? Something went wrong here, because you can't go from 100 to zero in three days unless you replaced all of your water.