I have to agree with gfk109. A few years ago I had problem with gnats and the best solution was to use sand to cover the tops of my soil; it works. Yes, a few will get in through the drainage holes, but they will be few and far between. I add "Great White" beneficial bacteria to my soil as well, so that helps too. I stopped using sprays like Azamax because the sand does such a good job. Most of the gnats I see now are dead on my floor, so I just have to sweep them up every once in a while. Keeping a clean garden also helps...