In my opinion, if your not gonna pick up the ADA plant substrate, and just that forest sand for looks, then I would find some comparable stuff from a local hardware store and be done. In terms of the dirt, I've used backyard dirt no problem without adding extras (just emmersed thus far though). I would figure you are looking to buy a bag of topsoil though? Make sure you aren't buying cow pies or anything with % cow pies, as that isn't desirable. A few inches of sand should cap the dirt, just don't go digging after the tank is established.