I suspect the anoxic zone begins much closer to the surface. IMO the bacteria living in the top layer of the rock would use up the oxygen very quickly. I've always had 0 nitrates with my porous, thin, loosely spaced rock, and I like tons of flow.
I'm not sure if any studies have been done on the distance oxygen penetrate live rock, but it would be interesting to find out. Where did you get 1 to 4 inches from?
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