I had an issue with fungus gnats in a couple house plants. To eliminate them, water the plants from the bottom and cover as much of the open top of the pot as y

I had an issue with fungus gnats in a couple house plants. To eliminate them, water the plants from the bottom and cover as much of the open top of the pot as you can in flypaper (yes, this can leave a sticky mess behind). They need both moisture and oxygen to reproduce, which means they only live in a damp top layer of soil. So leaving the top layer of soil dry prevents them from reproducing. And since fungus gnats are dumb and slow they fly right into the flypaper, removing themselves from the gene pool. You can also replace the top couple centimeters of soil. You'll still want to keep the soil as dry as possible for a week so any remaining gnats stop laying eggs.