If that is your purpose, to get rid of fungus gnats, you need to change your point of attack.
Lisa, you really need to figure out why you have so many fungus gnats in the first place.
They love these stinky, half rotted decomposing media and associated debris. I have been bothered by insects over the years but not by fungus gnats. I keep debris out of my pots, I keep the media fresh, I clean up plant debris from the entire growing area, no moss or algae on the side of my pots.
Good growing area conditions help your plants in so many ways. Just the installation of an oscillating fan can make a big improvement in the air quality and overall health of your plants.
