I suspect the CYA levels are higher then I thought, as the last test indicated. Since my Cl readings are so variable, I'm sticking to a schedule of adding 12.5ppm Cl every 12 hours (8am, 8pm). This should hopefully maintain at shock level long enough.

The pump is running 24/7, not backwashing though due to the pressure in the filter tank not being elevated. Pool Co. out to give me a quote on replaster yesterday indicated that my sand filter may need cleaning if I've got lots of algae that is difficult to remove. Any truth to that?