I'm running miracle gro organic potting soil below sand in my 125G. I've done it in pots in my 55G, and it worked *really* well, saw some amazing growth on the plants. It's a bit cheaper than "for aquarium" soil (similarly to how hydroponic submersible pumps are cheaper than 'for aquarium' pumps).
I'd highly recommend the soil. Make sure you get *organic* since some of the chemicals they use in non-organic products can be harmful.