01-20-2012, 03:18 PM
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Originally Posted by faronem
I've never heard that before...could you please explain the reason to let it sit?
Tap water may (and usually does) contain dissolved CO2, and that acidifies water so it lowers the pH. You often see this during a water change, all the bubbles from the influx of fresh tap water. Allowing a glass of water to sit overnight allows the CO2 to dissipate, and the pH reading may then be higher than it would straight out of the tap.
It depends upon the CO2 level in different water supplies; mine is negligible, I get 7 or 7.2 whichever way; but some people will see a difference of a couple decimal points. The higher reading after CO2 has gassed out is the true pH of the water.