Does high FC level (25 to 30ppm) have any effect on CYA test results? For the past couple days I have been trying to shock to eliminate .5ppm CC and have not been able to maintain constant FC level. (I've been leaving my pool un-covered so that I don't damage my autocover from high FC level). I made the assumption that this was due to something in the water eating up all the FC but on a whim, I just tested my CYA and it's basically at 0 (or at least below 20 which is where the gauge on the tube stops). Makes sense now that the sun has been burning up all the FC and is probably the cause for why I've been having to dump so much chlorine in over the past few days. I don't understand where all the CYA could have gone because just a couple months ago it was around 50 to 60. I usually don't test for CYA very often because it never changes much at all over the course of a full season. I guess I've learned a valuable lesson. But back to my original question... Is there a possiblity that my high FC level from shocking has thrown off the CYA reading? I'm off to the PS now to get more stabilizer!