I read that pouring a layer of sand on top of the soil in a pot would eliminate fungus gnats, reportedly because the sand cut up gnats or their larva, Nice thought, and widely believed to be true. But not so. Instead, use Bacillus thuringiensis israelensis (Bti) as was suggested very early on in this thread. Use the Bti (dissolved in your irrigation water) for at least 3 weeks to break the gnat's life cycle. Jean in Portland, OR