We have been trying to get rid of black algae in our white plaster pool since early September caused by having our CYA levels too high and FC levels too low for CYA level. Following the advice we got here we lowered our CYA level by draining and refilling the pool Then we shocked the pool very rigorously (FC 30+) until we stopped losing FC overnight and then let the FC levels drop. We didn't see any visible change in the little black spots we have all over our pool except we stopped getting smears when brushing.
When the FC got down around 18 we had a renewal of overnight loss so we re-shocked the pool(FC 20+) until no overnight loss.
All along we have been brushing 1 -2 times daily with SS brush.
When we dropped the FC level for the 2nd time we got no overnight loss so we have been maintaining the FC level at 8 - 15 ppm. Only exception is that 2 days ago the FC dropped to 6 ppm over the course of an unusually hot day but we raised it back up to 15 ppm that evening. There is no FC loss overnight.
The weather here in Sacramento has shifted so we have high 70's during the day but the overnight temps are down into the upper 40's. The pool water is pretty darn cold at this point.
We still have the black spots all over the pool. So we have some questions:
1 )Do we need to continue brushing the pool daily or is it safe to drop back to a less frequent schedule?
2) Will this continuous brushing w/SS brush over a long period damage the plaster or wear it down/weaken it?
3) We understand it can be hard to tell when you've killed the black algae because it can stain the pool. And that it can stay alive in the roots and rebloom from that very easily.
Assuming these are stains and not active algae spots (given that we aren't having any overnight FC loss) is there anyway to get rid of the stains?
4) Is there anyway to seal the plaster surface to keep the algae from re-blooming (assuming the roots are still alive)?
5) We were planning to put a cover on the pool for the winter but we're worried that when we take it off next spring we will find a disaster due to regrowth of the black algae. Is this a realistic concern?
6) Could some of the spots be stains from something else? Before we got advice here we had used Black Algaecide 3 times on the pool and have a small spot where there is blue staining on one of the steps. We also have some very small rust colored spots on the same step.
15,000 IG white plaster, Hayward 4 cart filter, Jet Vac sweep