Monthly is not frequently and it is plenty of time for the changes to happen. And once the change happens you have to be willing to do smaller water changes more frequently because a larger one will cause Ph swings. I just learned this the hard way (I only used my high ph test and as a result did not know that my Ph had dropped significantly in my 10 gallon tank, the Ph swing set off disease in the tank after a large water change).
Even if the whole thing was holey there would be some fizzing at the surface that you would be able to see. If you can post a picture of the rock someone might be able to identify it and let you know if it is the kind that will mess with your water chemistry.
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