Like others when testing for CYA the black dot never disappears! What is the concern over the test results not actually being zero? I don't understand.
You don't want your CYA to be zero. If you have an outdoor pool and you are manually chorinating (as oppsoed to using s SWG) then you want your CYA to be 30-50 ppm.
I don't know how it could unless you've just added CYA and it hasn't dissolved yet. A little cloudiness to the water can throw the reading off by 20 or 30ppm to the high side, but I don't know any way the test could read it too low.
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