Potting soil has too many nutrients in my opinion. Stick with low organic matter top soil (aka "dirt"). If you start out with Miracle Grow soil or vermiculite you're going to miracle grow algae unless you've got a ton of CO2 and plants, plus just the right amount of light. It is easy to add nutrients to a soil based substrate, but if you start with too many there is nothing you can do except frequent, large water changes until all the excess nutrients have leeched out of the substrate.
Besides that, when organic matter breaks down it can form Hydrogen Sulfide (the rotten egg smell), which is toxic to plant and animal life. Potting soil is mostly organic matter, usually peat moss, with small amounts of limestone and pearlite mixed in. I've always had excellent luck with just dirt from the corner of a wheat field mixed with a small amount of vermiculite and topped with some play sand (be sure to rinse it a couple of times). Anyway, just my two cents.
Last edited by smannell; 12-03-2011 at 05:56 PM.