Anerobic pockets, huge piles of filler material, and empty open spaces are all things im trying to avoid. Aquasoil does hold a slope very well. Ive been able to create some decent slopes by pouring into the tank slowly while misting with a water bottle. Over time however the water does win and everything settles. Ive have thought to attempt to grow the plants emmersed on the slopes then fill with water once a nice root structure is in place to hold the dirt. This method floated around for awhile using HC but i rarely read any positive outcomes.
Ive also though to create a sort of molded shell using stainless steel screen like that you see flame moss growing on. The idea is basically to have two layers about 1.5 inches apart to sandwich in some substrate. Thing is is that stuff is expensive and hard to shape without a mold.
Last idea is a hybrid of both. Create the hills while dry then lay on some semi rigid net like material (i know of no such product) over the slopes. Ive seen this method used to prevent erosion on a large scale but dont know how well its would work small scale in the tank. Something custom may be in order like netting dipped in an epoxy so it hardens up.
Well, all in all just brainstorming at the moment. If anyone sees any pitfalls or ways to make what ive come up with better let me know. In the end ill try something, im going to have to see it not work to believe it.