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?