Hello, first post TFP lurker here. The pool described in my sig came into my possession about two months ago. The previous owner had a pool service, but ran tablets and CYA level was at ~375 before I had the pool RO'd on October 8th. The pool has had some problems including very high pump pressure (cleaning the filter fixed that), extensive leaf stains (vitamin C tablets did the trick), yellow/brown dust all around the drain (after RO and shock, all gone now), still working on problems with the heating and solar.
Just before the RO I wasn't good about keeping the FC up due to being both new at this and constantly on travel- I was trusting the tablets left in the chlorinator to do their job. Sometime when I was away or wasn't paying attention the chlorinator ran out of tablets, and these little black spots showed up in a small patch around 4" across, about midway between the edge of the pool and the main drain. The test kit I had at the time showed 0 ppm FC so I dumped 4 gal of chlorine in and scrubbed the spots, but no luck.
Fast forward it's been about a month. After the RO I've been keeping the FC high, up to 20ppm when I initially add chlorine and no lower than 10ppm, to try to help fight the spots which I can only scrub when I'm home about once a week. I can't say I'm passing OCLT as the pool seems to consume about 0.5-1ppm per night and the spots are still there. They haven't shrunk but they haven't spread either, the patch looks exactly the same as when I first noticed it.
If the spots are indeed black algae do I just need to be scrubbing more often? The rest of the pool looks great. Admittedly I haven't explored the light niches yet to see if there's anything hiding in there.