Hard-to-defeat black (?) algae


Sep 4, 2009
There are a few places on the bottom of my pool that seem to grow a "black" algae, even during periods when I keep FC constantly above 4.0 (CYA is 62). Brushing these spots doesn't dislodge any of the algae (unlike the "green" algae I used to get before I drained my pool, which would send up a "dust cloud" when I brushed it); it just looks like a dark stain on the bottom of the pool that grows larger over time. Shocking and brushing seem to eliminate it temporarily, but when chlorine gets back to the 4.0 range, it starts coming back, always in the same spots on the bottom of the pool. I didn't think I needed to keep chlorine as high as 4.0 all the time, but even that doesn't seem enough. Water temperature has been 80 - 85 degrees, and the water has been remarkably clear, never cloudy in the least. Anything I can do to eliminate this problem completely?


TFP Expert
LifeTime Supporter
Feb 23, 2008
Denton, TX
Yes, you can. First, you can shock your pool and brush until all algae is gone, you have 0.5 CC or less, and you lose no more than 1 ppm FC overnight. Since your CYA is 60, your shock level is 24 ppm of FC.

After shocking and clearing your algae, you can start running an appropriate amount of FC in your pool. 4 ppm is not enough if your CYA is 60. The absolute minimum for you is 5. 7 is best. You are getting algae because your FC is too low. The water may seem clear but it isn't. It has algae spores in it and they are affixing themselves to your underwater surfaces and continuing to grow.