I would think stacking LR is more like the opposite of tetris - you want lots of holes and caves underneath, behind, between the rocks both for cover for the livestock, and for good water flow. If you pack it in as tight as you can, you're going to end up with dead spots in the rock pile unless you have insanely high flow to force water through the stack. Leaning against the back wall is fine, just try to keep it clear of the sides so you have room to scrape algae. I also recommend starting with a layer of eggcrate on the glass bottom, getting in a layer of rock then putting the sand in, so the rockpiles aren't directly on top of the sand. If they are, and you get something that likes to dig, you could end up with a toppled rock pile if there's any excavation under it.