"Holding Chlorine"

[Watermom]

That is a pretty big chlorine loss over one day. See if you lose more than 1ppm from sundown to within two hours of sunrise the next day. If so, you are fighting something in your water and need to hold at shock level until you aren't losing more than 1ppm overnight (and have no more than 0.5 CC). If you aren't losing overnight, then you probably need to bump your CYA up some.

[Cookieman]

So to see if I am not losing chlorine overnight would I test at sunset or after, skip the addition of chlorine, and see if it goes down before sunrise? When I have added at sunset or night, it has always been higher the next morning. If I find I am losing more that 1 ppm, are you saying I need to get it to shock levels during the night - as tested in morning? I did that two weeks ago to get rid of a green pool. Added enough bleach each night for FC to be 18-22ppm the next morning. Pool cleared in 3 days and I let it drop back down to best guess chart.

[waterbear]

Add chlorine at night, test (CC should be 0 ppm if there are no problems), test again in the AM. You should have less than 1 ppm FC loss and no CC in the AM. IF you do you need to get the pool to shock level and keep it there until the problem is resolved. Once you have done so you can determine how much FC you lose due to sunlight by testing in the AM and again at sunset to see how much FC you lose during the day. If it is more than 2 ppm then you should bump ypur CYA up in 10 ppm amounts until you minimize your FC loss. Remember to adjust your base FC level according to the best guess chart as you raise the CYA if that is needed.

[Watermom]

Just to add to what Waterbear said, after you add chlorine in the evening, wait a bit to let it circulate and then test.