I've used both garden soil--from a bed I'd amended with organic matter (leaves, lawn clippings, etc.) over the years--and commercial potting mix. The garden soil has worked well--so far (about three months). I don't know everything, but here's what I've found out so far.
I think potting mixes or soils can work fine, but I'd stay away from a "soilless" mix. Those are largely organic matter, which of course will begin to break down in the tank. Not quite as extreme, but it would be sort of like using lawn clippings as a substrate.
In looking for a mix I knew enough to avoid a blend with perlite or vermiculite (floaters), and I believe peat would probably add too much tannic acid to the water. The mix I chose included some "composted forest bark," but it never occurred to me how much tannic acid that would contribute to the water either. Even with extra charcoal in the filter I had months of tea-colored water, and of course since light doesn't penetrate though that as well as clear water, I also had some problems with plants not being able to get enough light.
Personally, the next time I use a purchased product I'll probably consider one of the "garden topsoil" mixes and amend it with about a quarter to a third sand. That said, I'm no expert and both my ill-informed opinions and my insightful observations are my own. You just have to figure out which comments belong in which categories.
potting compost? 
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