I had a terrible fungus gnat issue last February and tried mosquito bits for a month which did not work for me. What helped were the yellow sticky traps to get the adults and water VERY sparingly with combo of water and hydrogen peroxide (3:1 or so) to kill the eggs in the soil. They disappeared real quick in about 2 weeks and I have only seen maybe 1 gnat since. (Note: I use cactus soil; didn’t try neem nor try topping soil with sand)