If you are going to use "topsoil" you are better off with the cheapest, no-name stuff that you can find. With that you won't have to worry about additives or excessive nutrients, and you will save a few dollars.

As far as treating it is concerned, i soak mine for two weeks or longer, stirring it and changing the water. Since i started doing that I have had no substrate problems whatsoever.

I know that Tom says that boiling the topsoil is one way to get rid of excessive nutrients, and I'm sure he is right, but I wonder how that would work if the soil were very rich, like, for example, Miracle Grow Enriched Potting Soil (which, BTW, if used untreated will grow most kinds of algaes known to man, and quickly, too. Don't ask how I know that

Good luck.

Bill