I set up my 29gal a month ago. With black earth topped with play sand. Made sure it was no more than three inches deep.
Lots of live plants, easily 40 or more stemmed plants plus a few crypts and swords.
My concern, I've been getting air bubbles. I don't know if this is just pockets of air that got trapped while I was laying the substrate or if this is (and forgive me since I forget the proper term), pockets of gas.
I've gone at the sand twice with a chop stick, and it burps up a few bubbles in random spots. I did add about 40 baby MTS (baby as in 2-3mm) plus 3 larger adults.
However tonight I noticed some "gas" escaping on it's own and I wasn't sure if this is ok, or if I need to prod the substrate more often, or get more adult snails. Next time I get some plants from the LFS they are happy to toss me a half a dozen inch long trumpets.
It hasn't affected my girls (currently there are 5 female bettas and one nerite who has not moved). The only other thing of note are is the brown uh diatoms that are trying to kill my pennywort. Which is probably unrelated.