Join Date: Feb 2013
Location: San Antonio, TX, USA
I voted sand. I usually do 2 inches of dirt (shrinks/compacts over time) and 1 inch of sand. I have tried more than 1 inch of sand, but it actually makes it hard for the plants to reach the dirt, so 1 inch will do.
The problem with gravel is that it makes it a nightmare for foreground plants like HC, dwarf-grass, glosso, etc. It's harder for these plants to put a foothold. Nevertheless, gravel does serve it's purpose if all you want is bigger stem/root plants.