I have a 15000 gal in ground pool with a DE filter. I just started reading and learning from this site/forum. Its got great info but the chemistry and problem solving is a bit overwhelming. I had algea from the winter break and I used Kemtech liquid chlorinate to shock and my pool is crystal clear. But the grout between the tiles has small algea on it and a few big rain storms have thrown my chlorine outta whack. Just had my water tested at Leslie's and everything is within range but chlorine is a big fat zero. Do I need to add chlorine to shock levels or just to get the desired FC (3-4 ppm)? my CYA at the moment is at 60 and according to the chart my "target FC is 7" and shock level is 24 which means almost 500 oz of chlorine at 10%. How do I know what level CYA I want to maintain? Help is appreciated.