Sand would be fine. You'd want to use pool filter sand, but be careful of potting soil and the type you use. You don't want to use anything with fertilizers in it. Potting soil can be quite messy too. On top of this section is a thread on MTS. You may want to read that. I've never used "just" potting soil, but I do use MTS. The problem with most potting soil is that they contain fertilizers, that if leached into the your tank can wreak havoc. Many also contain a lot of wood chips that can make a mess of your tank as well. Also becareful of the so-called "organic" potting soils. They aren't always what they appear to be. One more note... I don't believe you want to use "potting mix", as this will surely contain things that you don't want in your tank.