We find this product effective in managing fungus gnats. 

Mosquito Dunk Active Ingredient 10.31% Bacillus thuringiensis

How to use:

Set 1 quarter of a dunk in 1 gallon of water and allow it to mix with the water naturally overnight. The dunk chunk may still be floating in the water, which is normal! Pour this treated water in your watering can and use it to water your houseplants. Repeat once per week to keep the fungus gnats away. Wash your hands with soap and water immediately after using the Mosquito Dunk, avoid contact with eyes and mouth.