I'd consider making PVC and eggcrate structures to stack the rock on. With a little creative rockwork, you can really hide the PVC and it takes much less rock to get some nice looking reef structures. You might ask Troy Valentine to show you his set up. It looks great. After I did this to my tank, I noticed an immediate improvement in flow in/around/behind the rock. And, there was much more of the sand bed exposed to the water; less detritus build up, more room for the sand animals to move around and do their thing. If you have a deep sand bed, you're relying much less on the rock for bio-filtration. Once I had the PVC/eggcrate structures, I had maybe 100-125 lbs in my 110 and it looked fine to me. If you really fill up a tank with rock, it's much harder to keep water flowing all throughout the rock work.