I found that BT can be used for that problem but I do not know the proper mix for use in houseplant application. My BT container is older and does not have the info on it. How should it be mixed and applied.
I Have Fungus Gnats in My Houseplant Soil
Certified GKH Gardening Expert
This article will help with fungus gnats: https://www.gardeningknowhow.com/special/children/fungus-gnats.htm
Neem oil will also help take care of these pests. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm