ok, for soil, i used this :
it's a soil-less hydroponics growing medium. it has all the good stuff a growing plant needs, bat guano, worm castings, dried kelp, bone meal, and a few other things. it is high in perlite though, but it drains excepetionally. and yes, i used this under my aquatic substrate as well.
for the aquatic sub, i needed to think about it for a while. i needed something that would look good, and natural, so black was out. i also wanted something small enough that "if" a frog ate it, it could pass it. i chose red sea florabase. mainly because it is, for the most part, clay. after it absorbs water, you can crush it between your fingers, just like kitty litter. so i get a couple by products here, i get clay for the plants, especially the crypts, and if a frog does eat it (doubtful), it should
be able to pass it.