Andrew, you need to think thru the steps at work here. The calcium carbonate sand is not really soluable at ideal reef pH, but becomes more soluabable as the pH drops, ie in the anoxic zone of a plenum or a deep sand bed. I think that the amount of buffering may be somewhat limited by the water energy thru the bed and the changes in pH depending on infusion rates and O2 levels.
My thinking is that a plenum based FOWLR would prolly show more stable alk /ph//etc levels, IF the fish load was not overbearing, the plenums substrate had a reasonabably bio diversity and the water flow and circulation rates where much higher than a typical swim tank with a few HOB power filters on it.
The main thing this scenario has going for it , at least from my point of view is that between the no demand for calcium, combined with fair outgassing of CO2 thru surface interface and or skimming, hopefully the CO2 will dissolve some carbonate and help buffer, esp since in a swim tank you have no mega calcium consumers to throw the calk,ph/buffer thing out the window
Once you jack up the light levels and add calcium consuming organisms it changes tho whole thing, and thats where the tricky chemistry thing comes into play
Not bad Doug, but the major thing to remember is that the high pH due to biological substrate activity is only in the DEPTHS of the sand bed. Once the dissolved CaCO3 begins to diffuse out of the deep low-pH sediments, the pH of the surrounding water increases and the CaCO3 again becomes insoluble, both taking out some of the protons (which has the net effect of raising pH) and precipitating out of solution, kind of a chemical vicious circle.
Once the biofilm forms on the substrate particles, little is available for dissolution into the water column, regardless of the pH (if your specimens are still alive, that is...)