I've been alternating between maintaining my own pool and having a pool service on and off for the last 14 years. In the prior years, I've always maintained CYA levels between 80 and 100 and kept FC levels between 2 and 5 PPM. For the last 3 years I had a service, but in March of this year I went back to doing it myself and due to CYA well over 100 I did a drain and fill in April. My pool is generally over 88 degrees in the summer and I'm accustomed to weekly maintenance but not daily maintenance. The solar cover has been off for over a week since the pool reached 95 and has yet to drop below 90. Since I started using the principles of this site in April and followed the procedure for adding borates via 11 boxes of 20 Mule team Borax in May, my pool water is better than I have ever seen it, but I started to notice an issue last month.
I have random black spots around 1/4 to 1/2 inch in diameter in different locations of the pool. Probably about 30 of them. After researching here, I believe it is black algae so Friday I brought the FC level up to nearly 20 and have been holding it between 16 and 20, running the filter non stop and brushing the entire pool with extra emphasis on the spots once or twice daily ever since. My PH was at 7.4 before starting and I added a quart of acid in order to bring the PH down to increase chlorine efficiency. After brushing and vacuuming today, these are my levels at 10AM:
BORATES appear to be between 50 and 80.
The spots don't appear to be changing. How long should it take?
P.S. I cleaned the filter with an Aqua Comb on Friday and today both before and after vacuuming.