planted tank sickness
Join Date: Jan 2006
Location: Marin County, CA
using something to hold the substrate back will help in the long-run. try using a long piece of wood to act as a barrier, or a big stone. but even with this technique, the slope will level out a bit eventually. you might have to move the substrate around a bit from time to time to keep it the way you want it.
the roots of the plants will also help hold the soil in a slope as the plants grow, just like plants growing on a hillside will help prevent erosion.
alternatively, you can build terraces in the tank with plexiglass prior to the addition of substrate if you want.