Speaking of surface area... it's not only depth of the bed, but grain size and composition. Smaller grains = more surface area, and different materials ALSO can add or remove from SA, being of equal grain size. A fairly simple and easy to understand comparison would be between "river rock" type pebbles (smooth, non-porous) and crushed lava (rough edged, porous). Generally, the smaller the grain size and more organic the composition, the greater the chances of impaction. For this reason, I'm not a great fan of using potting soils as substrate although a lot of people seem to do "okay" with them. I've yet to read or see or hear of a single potting soil tank that didn't require some pretty major overhauling after a few years in operation. The normal procedure is remove fish, remove plants, drain, replace bed, replace plants, test cycle, replace fish. For me, that's just too much blood, sweat, and tears every 3-5 years.