Bacillus thuringiensis israelensis is a bacteria that's really good at killing mosquito, fungus gnat, and black fly larvae. It's harmless to everything else and it's the active ingredient in "Mosquito Dunks".
You can break up a mosquito dunk into crumbles and soak them in a pitcher of water overnight to inoculate it with BTI. Water your plants with it to inoculate the soil and it'll kill off the larvae -- I go a little further and sprinkle crumbles over the soil in my pots to keep them inoculated.
It's my go-to for dealing with fungus gnats in plants and I've never had it fail.
Go to the "Filter lists" tab in the dashboard. At the bottom of the list click "Import" and paste the URL ( https://raw.githubusercontent.com/DandelionSprout/adfilt/master/LegitimateURLShortener.txt ) in the box. Then click "Apply Changes" to save it.