It does not have to be a special kind of topsoil. Really anything that doesn't have a ton of twigs or is fertilized with something like Miracle Grow would work fine. I've used soil from my backyard in a tank before, and didn't have any problems. Just mineralize the soil before you add it to the tank. And laying it down and capping it with sand should work excellently. It's a great substrate, and it's dirt cheap (haha, puns...).