i've had battles with fungus gnats. they are relentless. sand did not work for me. it didn't let the media breathe and was killing my plants. a quarter to half inch of perlite will work much much better to keep them out of your soil. i picked up a hot shots pest strip and cut it in thirds, hanging one piece in my tent to kill the airborne gnats, and gave one watering of azatrol at 2tbs/gallon to kill the ones in the soil. this is the only combo i've found to be effective, and i tried several.