I saw one example that used a volcanic mineral substrate as the base of the hill then put the aqua soil on top prior to planting.
I'm new to the hobby and want build up a hill as part of an aquascape in an aquarium. I saw one example that used a volcanic mineral substrate as the base of the hill then put the aqua soil on top prior to planting.  Is there a benefit, structurally or ecologically, to using a different type of substrate like the volcanic mineral substrate to build up the elevated areas, or "hills" in an aquascape if I am using Eco-Complete substrate? Could I just build up the elevated areas, or "hills" with just extra Eco-Complete substrate or am I missing something?