Agree with others that you cannot use anything under the substrate. Rock will be OK (leaving aside the question of mineral hardness here) but this will only serve to "replace" that portion of the substrate, and any substrate added on top will inevitably shift to level out so there is no real value in using rock to elevate a section of substrate.
I also agree that sand is probably the best material, with the provisos that it should never be white (this affects fish) and should be inert (not affecting GH, KH or pH) unless this is needed to raise the hardness/pH for fish requiring this. Play sand is the safest sand, and least expensive. Avoid any "industrial" sand like construction, blasting, etc, as this is angular and sharp and can damage substrate level fish.
As for "aquasoils," some of these may help plants, many do not. And they may affect fish. I won't get into the details, but just say that in most cases they are more trouble than value and best avoided, and unnecessary for plants anyway.