I think a peat/vermiculite mixture would be a good choice for substrate.  Peat tends to have 
alot of organics in it so over time you may produce some anaerobic zones in the substrate.  
Contrary to popular belief, some anaerobic breakdown of organics can serve the aquarium 
environment well as they tend to be highly reducing zones (for example, nitrates --> 
ammonium).  Anyway, I think a lot of how one is eventually successful growing plants in 
the aquarium comes with trial and error.  Also, be aware that the peat will leave the water a 
slight tea color.  Good luck!
Robb D. VanPutte
Dept. of Biology
Texas A&M University