I have had a pool for 3 years and with the help of this forum have kept it clear. I was not diligent about testing this year and developed a black algae problem. I am now testing and have been shocking for the 3 days. There is a very slow change. My big question is that for the last 2 years my CYA was about 90. I did not drain the pool but was diligent about testing and so kept the FC level high. This year I did not test but basically put in the same amount liquid chlorine as last year but ran into this algae problem. When I tested the CYA was 0! How could the CYA go to 0 from 90 the year before. How can I bring the CYA level up?
24500 gallon pool
DE Pentair filter
FC 0 (just yesterday I put in enough chlorine to bring it up to 12)