What youā??re experiencing is your tap water outgassing.

Iā??ll take a wild guess in that your water is likely well waterā?¦.at least water that hasnā??t been exposed to the air for long.

Outgassing isā?¦.de-acidification. So as the water outgases (de-acidifies) the Ph can only rise.

When I do a res changeoutā?¦.I must correct for the rise in Ph for at least 36 hours.
Airstones accelerate outgassingā?¦and I use a lot of air, so longer than 36 hours is certainly not out of the question.

Hope that helps,
HMR