You would probably better with top soil, rather than potting soil. Some potting soils contain perilite or vermiculite, which while not harmful, may float. The less organic material in the soil, the better. It is recommended on the plant sites/forums, that the soil be "mineralized" to remove the organics. info on that can be derived from one of the plant forums. Personally, I look for the poorest topsoil I can find, or even subsoil, and put a thin layer at the bottom of the gravel (mixed with the gravel) and a dusting of peat mosss, covered with clean gravel.
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