So I think we have black algae but am not 100% sure.
The brief history is that we bought the house with the pool (pool is >15 years old), water was a little cloudy, got the TF-100 test kit and discovered CH >800 and CYA was >100. Bruce and Sal did RO on the pool water and get the chemicals to a manageable level (CYA was 0!).
In trying to get the CYA back up to 30, added stabilizer to 15 and then started using pucks + bleach. Probably let chlorine get too low a few times but are now being vigilant because....
Black spots have appeared in the deep end and they're starting to spread reasonably quickly! It was finally warm enough on the weekend to get in and take a closer look. They're round but don't feel slimy. I couldn't scratch any off to try the smear-on-white-business card test. They are very hard, same texture as the plaster. I've been scrubbing at the spots periodically over the last moth or so but they are definitely increasing in number and spreading. I'd say the area covered at this stage is over 3 sq feet and the spots are reasonably spread out. Besides the black spots the water is CRYSTAL CLEAR!!
So here's what I'm doing. Adding bleach 2x a day to bring FC up to 16, and holding there as best I can, scrubbing like a mad woman with a steel algae brush 3x a day.
Current levels are:
Couple of questions:
1) Should I see CC with black algae?
2) Should I see FC loss with an overnight test with Black Algae?
3) Could the increase in number of spots be a consequence of me scrubbing the plaster TOO much? What color is the gunite underneath the plaster?
4) When I add the bleach I pour it directly over the area covered by the algae and let it sink to the bottom then scrub, then run the pump. Idea is that bleach will be REALLY concentrated on top of the algae. Is this a good idea or beyond stupid?
5) How long will it take before I see an improvement?
All advice will be much appreciated.
Will post photos ASAP.