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I though I heard somewhere on hear that the kH will measure differently straight from the tap vs. letting the tap water sit out for a little?
 

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This is the way I understand it:
PH can measure differently after sitting out. Also, it may change after aerating it.
It can go up or down depending on what additive the water company puts in to guard against pipe corrosion...
It's called "off gassing."
My tap water does it, but not every one's does.
When you run a hob filter it "off gasses" the water too.
That's why I prep my change water before hand so the ph doesn't fluctuate in the tank.
 

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Even if you use well water that has no additives added, the pH can read differently after a while if you let it sit, because carbon dioxide from the air will dissolve in the water and change the pH. The KH shouldn't change just by letting the water sit around.
 
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