I recently had to shock and once my FC stabilized, I was at 20ppm FC. I was hoping to be able to go swimming the next day but not at 20ppm. So I opened up the cover and let the hot sun shine down on the pool all day and by the end of the day, I was at 4ppm. I was amazed at how just 8 hrs of sunlight brought it down that fast however I was also concerned that maybe it wasn't just the sun and that maybe I still had a issue even though my FC had stabilized the night before. But sure enough, the next morning, I tested again and found 4ppm FC. So my question is, does FC burn off in sunlight at a higher rate in higher concentrations and then slow down once it reaches a certain ppm level? I don't ever recall losing that much FC during the day even with kids in the pool. My CYA is 55. Just curious how so much burned off so fast when I know I have stabilized FC overnight.