Well I wouldn't use soild from your own garden for starters...............

You can use soil but be sure it doesn't have any herbicides or pesticides or fertilizers in it or your asking for trouble. Plain topsoil will work but its light and will cloud you tank for some time. Diana Walstad is an expert in this area. Googling her shoudl yeild great results.
Here's a start.........
http://www.aquabotanic.com/plants_an...filtration.htm
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Member of the AGA (Aquatic Gardner's Association)
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