Black algae showed up last year... It depressed me. I scrubbed at it some and was sad. This spring I had had about 50 times the spots from the year before. Instead of sad, I got mad and thought I might try to actually do something about it.
A little background. The Ph in my pool loves to sit at 8.0. I got tired of the constant addition od MA over the winter so I let it go to 8 and stay. This spring, in preparation for swimming temps, I lowered the Ph and had these numbers
So, I added enough stabilizer to get CYA to 40 and kept the pump running 24-7. 2 days later, I donned the scuba gear and grabbed a pumice stone. It took about an hour but I swear I scrubbed the heads off every spot. Of course the roots were still quite visible. I got out and checked Ph and CYA. CYA was up to 40 and guess what! the Ph was still at 7.5. Last year it would have been back up to 7.9-8.0. I guess it took my plaster 2.5 years to cure...
Here comes the Shock. I dropped in 3 2.5 gal carboys of CL from ace hardware and another that I had left over from last year. About 4 hours later, at sunset, I checked CL and had:
Not a single spot is visible anywhere in the pool! I brushed all the walls down anyway with a SS brush with the fin on the back that really forces the bristles against the pool walls.
Today, things still look great. I did not do an overnight test, I'll do that tonight. The guys at the pool store wanted to sell me some algaecide but I wanted to try elbow grease first.
I am no longer sad or mad.