2nd summer of new pool and CYA too high


May 30, 2017
I completed my owner/build pool last summer and had little issue with the water chemistry (did not check CYA at pool start up nor during summer). Now this summer (hot summers in Arizona) I've been going thru a lot more 3" stabilized chlorine tablets than I did last summer and pool has had light green tint to it. My chlorine level would dip under 3.0 and I'd add a gallon of liquid Chlorine (from local pool supply). Several times it would require 2-3 gallons to get the pool clear with pump running overnight. In trying to get the root of the water problem I thought maybe it was the phosphate levels. I had that checked a few times and it was close to zero. Last I thought maybe the stabilizer (CYA) was too low and my chlorine was burning off faster than it could work. So I took sample to pool store and they tell my my CYA is around 110ppm. Reading the threads on TFP I know thats not a good thing and it explains why I've been struggling with a light green pool most of the summer.

First question, how did the CYA get that high? I do realize and its a lesson learned, without a reading at fresh pool water start up I dont know where I started. Is it possible my pool start up guy put too much stabilizer in the pool? Or would going thru six 3" tablets a week cause this?

Second question: I know draining my pool is the only way to lower CYA, but do I need to drain the whole pool or just some of the water? 20x40 rectangular pool, 3x5x3 depth around 24,000 gallons.

Thank you!


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Trichlor tablets add chlorine and CYA. That builds up. In Arizona, where we do not have rain and thus the water is never changed out during the year, the CYA builds up. I suggest you remove the tablets now.

You need to follow the SLAM Process. To do that, you need a proper test kit, see Test Kits Compared. I suggest the TF100. A proper test kit is needed to get the accurate water chemistry results needed to follow the TFP protocols.

While you are waiting on your test kit, add 5 ppm FC worth of liquid chlorine / plain bleach to your pool each evening with the pump running. This will replenish the FC lost each day to the sun and also inhibit any algae in the water from growing further.

Once you get your test kit and have reliable test results, a water exchange process can be discussed to get your CYA levels in range.


TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
NW Ohio
Six trichlor tablets a week would raise your CYA by 8 ppm a week. That adds up fast, it is entirely possible your CYA is way over 100 but tests usually top out at 100 (a pool store reading of 100 or more is usually code for "off the scale").

You are going to need to dump a lot of water, but without some more reliable numbers it is impossible to say just how much.


Silver Supporter
Sep 29, 2018
Mesa, AZ
Hello Neighbor,

1st, please pay attention to mcknauss. This member has provided excellent, consistent advice. Also, as a fellow Arizonan with my new pool, I have realized the insidious nature of Trichlor 3" Pucks. My pool builder utilized them at start up and soon after I started to see the results of CYA creep. According to the TFP App "Effects of adding" 6 tabs a week would bump your CYA 1.4 PPM PER PUCK! just as Donaldson stated.
To reduce your CYA, yes, a water exchange is in order for quick results and an immediate reduction in Chlorine target and minimums. Or, Like I have seen myself, with time, sunlight and pool water temperature, you could realistically see around a 10 PPM or more reduction in CYA per month in the summer.

Good Luck, Cheers!