For a big tank, eco complete or fluval stratum can really add up!
However, there are no rules. I just use inert gravel to create slopes simply because it's cheaper than using all aquasoil. For a big tank, eco complete or fluval stratum can really add up! Like Candi mentioned, aquasoil will also begin to break down after awhile (years), whereas gravel or volcanic rock will not. So it holds its shape a bit better long term.