Tap water is under pressure in pipes and generally has higher Co2 levels when fresh out of the tap, so the pH is slightly lower... when its allowed to stand for a while the Co2 will gradually offgas and the pH will rise back to its normal level.
Thats one of the reasons why some people experience more pearling in their tanks after a water change with freshly treated tap water, its like the tank getting a quick boost in Co2.![]()
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