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I have a 25,000 vinyl pool that is about half empty. I'm trying to lower my cya and currently it is reading 140. If I fill the pool up now will that dilute my cya? If so, does anyone know of a formula to best guess how much?
fc - 5.0
tc - 5.0
cc - 0
ph - 7.3
alk - 58
cya - 140
It will reduce it by however much water you replace. If you replace 50% of the water, the CYA will drop by 50%. Be aware that CYA tests aren't very accurate above 100ppm, so yours may really be higher than 140.
It might reduce it by less than 50% since excess CYA can deposit in the plumbing and can redissolve when the levels drop. Fill the pool, wait about a week, and test it again. Then if you need to lower it more just do some partial drains and refills to get it where you want it.