By "Generic Potting Soil", I think "Simple Potting Soil" could be used as well. Some commerical potting soils will change during their production year, as different products become seasonably available.
A Simple potting soil would have a moderate amount of composted organic matter (humus, leaf mould, etc) with some not-yet broken down material (peat, some tiny bark pieces, etc) and some inorganic material (sand, clay, tiny stones).
Its easier to mention what is NOT recommended. Because these ingredients add cost for the manufacturer, generally they're found in "name brand" potting soils.
Fertilizer - Miracle-Gro brand includes a slow-release fertilizer in their potting soil, intended to feed container plants, window boxes and such for the summer (about 3 months). This amount of nutrients will wreak havoc on your tank parameters, and probably result in an algae mess.
Perlite - while not a bad thing for houseplant soil, this stuff "lightens" heavy soils, especially those that tend to be high in clay. It provides porosity and helps to allow container plants' roots access to air from the small spaces between particles (very important for terrestrials). For the aquarium keeper, this stuff floats, and is a royal pain in the @$$, as the little white bits will float up for months and cling to anything in the water surface.
Vermiculite - also generally a good ingredient in houseplant soil, this is a mineral that has been expanded by exposing it to great heat. While it also "lightens" soils to some extent, it also provides a lot of surface area for water to cling to, and helps absorb and retain water, while not staying thoroughly WET. It evens out the wet/dry cycle. For the aquarium keeper, this isn't as annoying a floating component as perlite, and may help against compaction. (*aside - I've actually added some vermiculite to a soil underlayer as an experiment, with very good results to date).
Wetting Agents. These are surfactants. I've personally had a very bad experience with these when trying to pot up some very rare terrestrial plant cuttings. My bad experience was with Martha Stewart's potting soil from KMart. Shredded sphagnum peat has an annoying habit of being difficult to wet once it gets very dry. It actually repels water to some extent. A wetting agent, or surfactant, gets the water to make contact with the other materials, and increases absorbability. But, it also increases the moisture retaining time. For me, this kept the precious cuttings too wet for too long, and caused rot. I'm not positive what the effect would be in an aquarium, but my sense is that it would not be good for the creature or plants. Plants have a very thin natural cuticle to protect them, and I'd guess this would not "play nicely" with that. Who knows what the long term effects on fish would be.
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