look up walstad method planted tanks, diana walstad is the moderator on the forum you were directed to i believe, she also wrote a brilliant book called "ecology of the planted aquarium" which is a must have IMO. I use miracle-gro organic choice potting soil (in the brown bag) in most of my hard water tanks, the main risk is acid crashing due to acid generating reactions in the soil if your water isn't adequately buffered. I also wouldn't use phosphate buffers here because its not the natural order of things. But for hard water tanks potting soil works very well IME. It just smells really bad when you're tearing down a tank