My P.O.P. is gone. need help again

[kelemvor]

Sounds to me like your fc is being eaten up by something rather than getting burned off by the sun with a 100CYA. I would shock to 25PPM. Keep shocking until you lose less than 1ppm FC between sundown and sunup.

You might also want to get some distilled water and mix that with pool water while doing your CYA test. Half pool water and half distilled, then double your reading to get your actual CYA number. I know my test kit only goes to 100PPM so if yours is the same, it's possible you have more than 100PPM CYA which would affect how much chlorine you need in the pool for normal use and/or shocking.

You should avoid using any dichlor or trichlor as those will increase your CYA level.

Edit: I see in your other posts that you recently tried a partial water replacement to reduce your cya level and it didn't budge. I think that probably means you were much higher than 100 and you'll have to do more water replacement than you already did to get it down some.