best fix for fungus gnats, get some sticky paper to catch the adults.. then cover your soil with sandbox sand.. like 1/2-1" will do just fine.. the sand is so fine they can't get into the soil to lay their eggs.. within a few days the gnats will be dead and no offspring because they cant get into the soil through the sand to lay eggs.. i grow my garden indoors in the spring so i have good plants for the summer. tomatoes, brocolli, watermelon, cucumbers, etc. had some issues last year and put sand down on the soil.. in a few days they were gone and no more came in their place.
hope this helps..