So to follow up my old CYA thread, I'm doing incremental water replacement by pumping some water out to irrigate the lawn then replacing (dropping about 3" at a time from an approx 15 x 30 ft rectangle, then refilling with fresh water ... yes I know this will take a while but with water rationing in place I don't think we could do it in one shot). I've been aiming to keep the FC at around 10 but know there's some residual mustard algae because every morning when I brush the pool there are some light clouds of yellowish-brown dust and my FC has been dropping from 10 to around 7.5 overnight.
I see from the FC/CYA chart that the recommended shock level for a CYA of 100 is 39 - now when the note at the bottom of the chart says " lower FC levels are often sufficient though they take longer to kill algae" what's a ballpark lower number that would probably work - say, 20-25 ppm? How long would I probably have to keep it at shock level to subdue the algae (and hopefully reduce the frequency at which I need to add a gallon of bleach to the pool?)
For the past few days, I've been adding a gallon of 10% bleach every morning and it seems enough to keep the algae from increasing but not enough to kill it.