Join Date: Jul 2003
Location: Charlotte, NC, USA.
I'll let others chime in, but basically you want the cheapest dirt you can find. Miracle Gro has extra stuff to help your plants grow. Good for potted plants, not as good for the tank since they can leach out. See the nitrate, ammonia, nitrogen, phosphate and potash.
Note - I haven't been involved in a soil tank for 8 years, so other stuff may be "right" these days.