We must be misunderstanding how this all works, because I didn't think we could get algae if our FC stays within the target range indicated by our CYA level.
This is the scoop. Our CYA has been between 50 & 55 this past month. Our CC level has stayed at 0. Our FC has been high since we raised it to 27 back on Aug 13 when we started an algae bloom. We kept the chlorine at that level until we didn't lose chlorine overnight. Just to be safe, we then kept it at 27ppm for another 48 hours. Since that time, we haven't added any more chlorine because we were letting FC drop to normal levels. We didn't notice any more algae until 8/20.
On that date, our FC tested at 22. Our water was clear, but the bottom had a green powdery substance that dissolved when it was touched, no scrubbing was needed. There weren't any spots on the sides of the pool. The vinyl & steps weren't slimy - they just had that green powdery residue. Since the FC was still so high we assumed that what we were seeing in the pool is the pollen that everyone on this forum has been talking about.
We continued to vaccum & backwash as usual, but each day we'd still see that stuff on the bottom of the pool. We didn't add any more chlorine, the FC still hadn't dropped into normal range yet.
By 8/31, our FC was at 12. It held at 12 (we had the solar cover on) until 9/5 (two days ago). We still were seeing the green stuff, but the water was clear & CC was still 0. Yesterday (9/6) the water was cloudy & the pool was covered with a full bloom of green algae. There isn't any on the walls, it's not yellow, but it's definitely slimy. The FC level had fallen to 6. We are now adding chlorine to shock levels. We keep it at that level for 24-48 hours PAST when it has held overnight without the FC level dropping.
So...what are we doing wrong? Why do we get algae even when our FC is within the target range indicated by Chemgeek's chart? Our solar cover has been on a lot. Will that cause it? This is the first year we used the BBB method & haven't used algaecide. Now we're wondering if we should start using it again.
By the way, our Ph is at 7.3 and our TA is at 110. Both PH & TA have stayed within normal limits this past month.