CYA test

If the black dot never disappears, you have little to no CYA. This means your chlorine will drop pretty rapidly when the water is exposed to sunlight.
 
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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