I discovered about a dozen black dots/domes on the weekend around the quarter the size of a fingernail which were blending in very nicely with my black pebbles. About half were on my top step. Because I nearly always swim at night, I hadn't noticed them. I scrubbed all the ones I could find down to a dark green smear and shocked the pool so FC is 20ppm (CYA is 60ppm). I have since been scrubbing every inch of the pool several times a day with a new (decent) scrubbing brush with good strong bristles. Visibly things are looking brilliant but having read the other threads I know not to be complacent - I continue to scrub the whole pool several times a day and keep the FC at 20ppm. I haven't seen any dramatic falls in FC though my measurements at that level aren't accurate - I'm diluting pool water half and half with tap water and reading 10ppm (if I use straight pool water it is way off the scale). I realize this is not a good way to measure FC but I figure the FC must be up there somewhere. pH was 7.6 prior to shocking - I added some acid on advice from my pool store and it is probably around 7.0 - I won't be adding any more until I lower the FC and get a new measurement. TA was 150 and CH 138.
What I'm wondering is... now that everything visibly looks fine, how much longer should I maintain shock level and scrubbing to ensure it's all dead? I propose the following plan:
20ppm FC for another week scrubbing several times a day
8ppm FC for a week after that scrubbing once a day
5ppm FC thereafter (minimum 3ppm or 5% of CYA), scrubbing weekly
I've always been quite obsessive about my pool but I was led to believe that 1ppm to 3ppm was a good range of FC regardless of CYA. I've since been enlightened! My FC was usually around 3ppm but it did drop to 1ppm sometimes which is how I must have got the black algae. My useless old scrubbing brush probably didn't help matters - I may as well not have been scrubbing at all.
Anyway, does the plan look good enough to be rid of it for good or should I be attacking it for longer at higher levels to make sure?