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.
oh WOW are you frickin serious???? since you say you only did it in pots, did you cover it with anything (IE sand)? my idea is to make a 'plant bed' approx. 2" thick, then cover that with about 1" of pool filter sand.
do you have a link to your 125? did you have to do anything special with the soil before adding sand over it??
going to try this in a 2.5gallon i have as well, put a beta in it and put it on my desk. will take pix tonight and start a journal of it as well.