Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
With no livestock it is not a problem, but some potting soil has fertilizers added that become available to the plants when the soil is wet. For house plants that is just occasionally, and the fertilizer releases slowly over several months.
In an aquarium the fertilizer is constantly wet, and may dissolve into the water very quickly. Perhaps even too much for the plants. Certainly WAY too much for any livestock.
I think I would use a cap over a potting soil that has fertilizer. This will slow the release of the fertilizer into the water column.
As for how much to use, if it is a smaller tank, and you are planting smaller plants you could get by with as little as 1" of soil.