Carl, Watermom, Janet, or Ben -
I have a question about the timing of taking an end-of-day FC test and when to take it the next morning. To end the shock phase, I know I want a FC that loses no more than 1ppm overnight, but when does "overnight" start and end? From reading other posts, I thought the last test of the day can be taken whenever the sun is off the pool and the first test the next morning is to be within two hours of sunrise.

This evening, I did a test at 7pm and the FC was 25.5. I thought that was late enough to be my final test. Out of curiousity, I tested again 3 hours later at 10pm and was surprised that the FC dropped 4ppm (from 25.5 to 21.5). I added almost a gallon of bleach to bring it back to 25ppm, but this got me wondering.

Here're my questions:
  • Why did the FC drop so much between 7p and 10p?
  • Is it because the FC is still fighting nasties in the water? The CC at 10p was 1.5, so I think this also is a clue that the shock level of bleach is still needed.
  • The sun didn't actually set until 8:38p tonight - could the UV rays hitting the pool indirectly between 7p and 8:38p also have contributed to the drop in FC?
  • The water is so invitingly clear, how many more days should I be prepared to continue frequent testing and keeping the FC at shock level (i.e., 25ppm)?

Sorry to be hounding you with so many questions - I'm curious about the chemistry and I've also got several eager swimmers attracted to the sparkling water.