Bti is recommended as a specific for fungus gnat control. You can buy it in granular form off the shelf by Bayer Advanced as "Mosquito Killer" (may no longer be available). You can also buy Mosquito Dunks. I just break off a chunk of a Mosquito Dunk and put in in a water container at least overnight and top water my plants with it. You can also just put a small chunk in you water tray.
Hasn't harmed any of my plants yet. Do not use with Utricularia as the larvae are a good food source.