Not using pucks or a lot of shock but CYA keeps going up


New member
Oct 5, 2015
League City, TX
I don't understand why I'm constantly fighting high CYA after numerous drain and refills, stopped using the chlorine pucks and started using liquid bleach. Shock the pool about every month or two and my CYA still is high (70). I didn't think anything besides the pucks and the shock added CYA. Any help would be appreciated.


TFP Expert
LifeTime Supporter
Jan 6, 2010
San Dimas, CA (LA County)
The CYA test is affected by temperature. So if the water is warmer now than it was the last time you checked the CYA, it could be a better reaction so it reads higher than it did.

Testing indoors versus outdoors also can make a HUGE difference.

How much is "not a lot of shock?" According to poolmath, if you use a 1 lb packet of dichlor, that will add 4 CYA to your pool. Given the subjectiveness of the test, just 2 packages could jack the CYA by 10.


TFP Expert
Mar 2, 2011
Do pool or spa water samples have to be at "room temperature" before testing begins?

With one exception, no. Our lab has run the pH, total alkalinity, calcium hardness, free chlorine, and cyanuric acid tests on known standard solutions at approximately 104°F, 75°F, 60°F, and 40°F. Only at 40°F did the CYA standard solution test higher than its actual value (after accounting for test variability). All other tests were unaffected by temperature differences.


Silver Supporter
Aug 28, 2012
Houston, TX
I would concur that you answered your own question when you stated you added "shock" to the pool "every month or two". You need to know exactly how many PPM of CYA is in ea shock package.

First, in pool chemistry, esp when the water is warming up, the difference btw one month and two months is huge. So, you need to document and be specific on the actual times you shock the pool over the exact time period.

If you test what w your CYA is, then add shock, you will have to increase your CYA level by that much, minus any major water losses or water mixes [ie, 12-16" rain in 1-2 days].

Keep good records by writing down dates and specific quantities. Also, as Rich suggested, the CYA test is very subjective. Just do it the same way [time, conditions & methodology] each time to be consistent.


Bronze Supporter
May 27, 2014
I'm reading this thread as someone who is still a learner; not a teacher, but doesn't TFP teach it's best to shock with bleach or other liquid chlorine? I know that dichlor and cal-hypo are not forbidden, but isn't it best to shock with bleach unless one is trying to concurrently raise CH or CYA and shock? Even if for no other reason, to keep PH from getting out of range?


Well-known member
Jan 28, 2014
Buckeye AZ
It's best to keep proper FC levels and then you NEVER shock. Pool stores get people in the habit of shocking once a month, or week, or whatever vague timeframe, to spike the chlorine temporarily. For a properly chlorinated pool (daily levels), you'll never need to shock.

IF you have algae or other contaminants, or ammonia, then you would SLAM the pool, keeping the FC at shock level constantly. In this scenario, yes, it is advisable to just use liquid chlorine, so you are not constantly juggling effects of other additives like CYA, which will keep moving the target.

And in general, it's very hard to remove CYA once it's in the pool, so better to start a bit low and ease your way to your target, once you are done clearing any algae issues.