This link from GWAPA is a good read if for mineralized soil substrate. You can substitute the soil with earthworm castings and put it straight into the tank. Remember to plant heavily from the start and use low lighting at first. If you want to use topsoil, get the cheapest one from Home Depot or Lowes that does not have any other additives to the soil. You'll have to spend time picking out stones and wood chips from it. I used something like Earthgrow brand in the past.
http://gwapa.org/wordpress/articles/mineralized-soil-substrate/
http://gwapa.org/wordpress/articles/mineralized-soil-substrate/
