I set the rocks on the glass bottom of the tank, build the structure up with rocks and aquarium silicone, let cure, then add the gravel and fill er up. Works very well, a 55 i've got has a rock wall built into the back with driftwood in it that was done this way. It's all the way to the top, and very stable. Plus, the digging behavior of malawi's won't disturb it because the rocks are set on the bottom of the tank, not on the gravel.