Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Do not use sand (or anything like it), Soil Master Select or any related material (Turface, Oil-Dri...)
I would look into a substrate that is more like flat little pieces, not rounded. Perhaps one of the Florite products (not their sand).
Even then you might find that hills and valleys are more stable if you can make any sort of retaining wall. Driftwood, rocks or a strip of plastic under the substrate can hold it in place better, especially if you will have a lot of water movement.
Another option is dry-start. Without water currents to move the substrate around you can get the ground cover well started and holding the hills in place, then add water.