We've been all over this site looking for an answer to this problem. We have a 5 week old pool with grey plaster interior, 19K gallons, and SWG. We have the test kit recommended on this site, the K2006.
Problem: Spots about 1 centimeter, but varied in ,that are brownish to blackish and circular and most have a smear look to them, similar to a small comet-like tail. Brushing does not remove them.
What has happened so far:
Pool was finished and filled with water and builder put one bag of granular chlorine shock in it, along with 2 jugs of muriatic acid. Builder told us we were all set until salt goes in 3 - 4 weeks later. At 3 weeks we noticed that it was turning cloudy and slightly green. Noticed first spots at this time also. Shocked with 2 bags granular chlorine. Pool and spots cleared.
Salt went in 2 days later and SWG was started. With SWG on at 50% there was 0-0.5 FC in the pool. Spots began appearing again. We assumed algae problem and added chemicals to achieve the following levels:
FC 27 ppm
CC less than .5ppm
water very clear sparkling
Pool passed overnight chlorine test.
During shock to 27ppm and constant running of pump all spots disappeared. Assuming we had beaten the problem, we allowed the shock to start to fade. After turning the pump off and the one day fading of the FC to 18, spots began to reappear. Brushing does not remove stains. High chlorine levels or chlorine placed directly on them will work. Vitamin C tablet removed them as well, but they could have just been scrubbed off using the tablet. We aren't sure if it is the same spots coming back or new spots in the same area.
We have many Texas Live oak trees in our yard which are putting out all kinds of things that end up in the pool, due to it being spring. The stuff is coming down fast and we are unable to get it all immediately cleared even though we run our shark, brush, and skim twice daily.
Question: What the heck are these things? Algae, tree tannins? How can we get rid of these?