Rocks should never be stacked on sand or gravel - that is asking for the stack to topple. Vacuuming, a digging fish or invert, etc., can undermine such construction. Rocks need to be built up over a firm base. I use eggcrate light diffuser and/or blueboard styrofoam. Either will diffuse pressure points and protect the bottom glass.
If you are building a major structure, build by the same priciple by which dry stone walls are built: Two over one, one over two. That means that if you have two smaller rocks, you bridge over both in the next layer with one larger rock. If you have a large rock, it should be supported by two (or more) smaller rocks. The larger rock can in turn support two smaller ones in the next layer, but they should be secured in the next layer with larger ones, bridging to each side, not a vertical stack. "Stacking" is exactly what you do not want, what you want is to almost weave the structure together with rock.
My mbuna tank was set up in this manner, several hundred pounds of rock, all the way to the water surface in the back. It never shifted in the approx. 15 years it was set. At the rear of the tank, a strip of eggcrate protected the back glass from scratches and pressure points, ending below the top row of rocks which did not touch the glass.
If you have never looked at a fieldstone wall, study one some time - a huge range of sizes and shapes, all tied together without morter and stable for decades at least. You just don't want it that regular and flat across the front.
