Read what he said again paying special attention to the 'why' of his statement and then ask yourself if you want to slather some man made chemicals all over something natural.
That's pretty rude when I was asking a realistic question! Just bc it's natural doesn't mean it can't seep something into the water to change the pH. Some "natural" stones do that and I would think clay would absolutely change your water too. Now I don't know if it's enough to matter or not!
Last I checked silicone is fish safe so I was thinking maybe it should be sealed using it.
I also know that the 3D backgrounds that people are making out of cement need to be soaked several times before the pH is equalized.
So please excuse my lack of experience with using such items that don't normally belong in an aquarium but let the man answer the question that was addressed to the OP!
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