With a CYA of 40, the FC must stay above 3ppm all the time - I'd target no lower than 4ppm for regular maintenance. My guess is the same as Charlie_R's - this pool has seen a few low FC excursions and has a nascent algae bloom. Use bleach to bring the FC over 15 and keep it over 15 until one day after all these things are true: The pool loses less than 1 ppm FC from sunset to sunrise. The CC is nearly 0 (less than one drop after adding CC reagent). The pool is clear. then let it drop down to no less than 4 ppm.