SLAM if CYA is zero?

You should test your pool water in the same location and lighting as the standard solution.
Bold mine
! Make sure to check (compare) the standard solution in the same situation you will be testing your pool water CYA sample
  • (no need to dispose of it, you can reuse it, be sure to label it clearly)
  • And it's suggested to "test" the calibration sample in different lighting situations
  • Find the lighting/area where you see 50 "easily" or every time
note: be sure your pool water sample (and the "calibration" CYA sample, for that matter) is over 60F (allow it to come to room temperature), as the CYA reagent is slow or even nonreactive under 60F.


LifeTime Supporter
Aug 19, 2014
Pacific NW
Yes lighting is VERY important. Best is direct sunlight outside, with your back facing it.
When I first did a cya test I had not known this and was getting way off readings using a lamp as light source inside.