So I know in theory how I separate substrate should I decide to use two different types of substrate in a set up, but how do people use substrate to create different levels or mounds? When I try, my gravel and sand mounds or levels quickly disintegrates so that the original effect isn't all that obvious. Also, just how deep can you go? I've been looking at some of the set up guides on the CAU website and they have some really deep (tall) mounds of substrate. They do seem to use different layers of substrate and at times, rocks to hold them in place, but when i look at them my head says "gravity will drag that down", yet it doesn't seem to! Is there a secret I'm missing? Edited for grammar!