My guess is that between your rocks and your substrate your gH is slowly increasing, which is also raising your pH... That's just my guess though. If there's alot of evaporation going on with your tank that will also raise the gH over time...
To me the ideal pH and gH 9x/10 matches your tap water- so I'd do some daily partial water changes (PWC) and in a week see how the parameters compare?
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