Just coming off a nasty fungus gnat infestation. I bought this product, and 4 days later I am gnat free!
https://www.amazon.com/gp/product/B01FRUT3FA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
It is a super concentrated BTI liquid. BTI is a bacteria that produces a toxin that destroys certain insect larvae (popular for mosquito control) but is perfectly safe for mammals, fish, plants, etc.. This product is a liquid with the appearance and consistency of a chocolate shake, and 5ml treats 2,000 gallons! Crazy strong, lovin' it...