Active member
Jul 2, 2021
Vancouver, WA
Pool Size
Hi All,

My pool is 17,415 gallons. 11 days ago my CYA was 42. Today I tested (at the store; I’m ordering my own test kit today) and it was 54. I’ve done a backwash, added water AND only added two Trichlor tablets in those 11 days.

Can two Trichlor pucks add that much CYA in 11 days? Seems improbable.

The only thing I did differently this time was I filled the water tester bottle a couple feet down instead of using surface water. Could that make a difference?

Here is a link to my Pool Math logs.



Bronze Supporter
TFP Guide
Jun 26, 2012
Edmond OK
The most likely cause is flawed testing. CYA is the one test that pool stores are the worst at, while simultaneously being perhaps the most important. This why we recommend doing your own testing.


Gold Supporter
May 20, 2020
Katy TX
Pool Size
Liquid Chlorine
According to Pool Math, for 17,415 gals, 2x 8oz pucks of trichlor (total 16 oz and fully dissolved) will add 3.8ppm of CYA.
The problem with the pool store CYA test is that it is usually very erratic. So there is little faith in the test result.

Also, it is always best to pull your pool sample water from about 18 inches below the surface, away from the return. 18 inches is a little higher then your elbow.

If you purchase one of the recommended test kits noted above, I suggest you follow this test procedure for CYA
  1. Make your 50/50 solution using your pool water and the R-0013 reagent.
  2. Mix it and let it sit for 30 seconds per the instructions
  3. Rather than looking down the tube at the black dot and pouring your mixed solution into the tube. I suggest you squirt enough in your tube to a specific mark - such a 100. Then look in the tube for the black dot. If you see it, then squirt in more mixture to the 90 mark, Then look in the tube for the black dot. Continue this for each decade graduation ( i.e. 80, 70, etc.)
  4. Once you cannot see the dot then the previous mark is your CYA level. So if you saw the black dot at 50 but not at the 40 mark then your CYA is 50.
This way you are not staring at the black dot constantly and seeing it regardless. I remember my teacher in grade school had us stare at a black dot for 60 secs and then look up and we saw that black dot everywhere we looked - its the same effect if you stare down that tube. You will see that black dot even if it is not there.

Also - Stand outdoors with your back to the sun and hold the view tube at about waist level. If sunlight is not available, find the brightest artificial light you can

Note- this will all make sense when you get your test kit and do your first CYA test.
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