Quote Originally Posted by TCP007 View Post
BigDave: No water change, but yes... I stopped adding granular dichlor since I super-chlorinated with liquid chlorine in late July. A mod here recommended I stop adding sanitizer until my FC was at or below 5ppm. I actually added my floater back to the pool in mid August because I was worried about not having any sanitizer whatsoever. The floater holds about 1-lb of 1" trichlor tabs that typically dissolve in about 1 week. But that's the only sanitizer I have added since late July until about 2 weeks ago. I test my water every Saturday, and my FC was still at 7ppm the week prior to this last, minor outbreak.
1 pound of Trichlor tabs in 35,000 gallons would only be 3.1 ppm FC of chlorine (and 1.9 ppm CYA) so if that's the only chlorine you've added from July until 2 weeks ago, that wasn't nearly enough since that's only 0.44 ppm FC per day. More usual daily chlorine loss is 2 ppm per day. Do you have a solar cover? Is your water temperature cold?

I suspect that your chlorine level was low for an extended period of time and that allowed algae to grow. Once algae is established, however, it takes higher levels to kill it which is why we recommend shocking the pool to higher levels to kill off the algae.