Why is my aquarium pH so high?

A bit puzzling because a limestone based substrate or decoration should only ever raise the pH to around 8.0. Once the pH gets that high, the rock stops dissolving, so there is no more change to the pH.

But I do also suspect there is something going on with the substrate? Do you have some non aquarium ornament in there or something like that?

Ian

Its not the driftwood as the tannins present in it will lower pH

New water, a dirty tank or unwanted chemical substances can cause pH levels to fluctuate.

You have something slowly dissolving in the tank. either rock, ornaments or the gravel.

Strip the tank and only then add one item at a time waiting for the pH spike in this way you will identify whats going on

I think you got a bad reading or are using a wrong range of pH test, or don't rinse out the test tube correctly, or you use a crappy paper test. I just don't believe the test.

either way it's probably fine if you just do proper 25-30% water changes every week.

note: read @ianab and melanie's answers and check if you're using the appropriate decorations and gravel.

It's probably the decor.