Place yellow sticky cards in the grow area (keep the sticky yellow cards up until Fungus Gnats are completely gone)
Get a fan blowing air over the top of the growing medium
Don’t water your plants for a few days

Kill the Larvae in The Soil:
Water your plants with is something that contains “BT” bacteria
Water your plants with 1 part hydrogen peroxide(3%) to 4 parts water
Avoid overwatering your cannabis plants
NOTE: Avoid using nervous system insecticides, such as malathion, Dursban (chlorpyrifos), and Orthene (acephate). They are labeled for use on many shade trees and ornamental plants for aphid control, but are not safe to use on cannabis. If something isn’t safe to be used on edible plants, then chances are it’s not safe to use on cannabis.