Randy Holmes-Farley
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TLDR: GFO seems to remove PO4 much faster in a tank with coraline-covered rock than in a tank without coralline-covered rock.
I'd certainly expect that. Bare calcium carbonate surface bind a lot of phosphate that makes it much slower to drop (or raise) the water levels of phosphate. in a bare rock tank, that might include far, far more than is in the water. maybe up to 50 ppm on the rocks for only 0.2 ppm in the water.