Quote Originally Posted by Sprocket
Friday just before taking 15in of water out 24FC - CYA 65

Sunday after water put back in 11.5 FC - CYA 42 added another 6 quarts of bleach , now pool was about FC 17.5 total or so , did not recheck after adding 6 quarts .
Just for completeness, the 24FC - 65CYA combination has 0.265 ppm disinfecting chlorine (HOCl) while the 17.5FC - 42CYA combination has 0.324 ppm which is not a huge difference. So either the "cutoff" for when a much faster algae kill occurs is somewhere closer to the 0.3 ppm range (rather than 0.5 ppm as I had thought earlier) or you were just hitting the alage hard for a week and it just coincidentally overcame the algae around the same time you decreased your CYA.

The "rule-of-thumb" for hitting a 0.3 ppm disinfecting chlorine (HOCl) level for shocking would be an FC that is about 40% of the CYA level. Still high, but not quite the 50% needed to hit the 0.5 ppm level.

Richard