As I understand it, the fungus gnats you see and even kill are not the main problem. You want to kill their root-eating larvae in the soil. Pesticides effective against the flying adult gnats are not effective against larvae. If you see gnats, you should presume your soil and roots are infected with larvae.
Consider adding to your feeding one of the live Bt bacteria products, commonly used for mosquito larvae control, much the same as other beneficial microbes are added. These are safe, 'natural' and effective. I add it at 2 mL/L, a low dose, with every feeding to prevent, not treat, fungus gnat (and any other) larvae from getting established.
Consider adding to your feeding one of the live Bt bacteria products, commonly used for mosquito larvae control, much the same as other beneficial microbes are added. These are safe, 'natural' and effective. I add it at 2 mL/L, a low dose, with every feeding to prevent, not treat, fungus gnat (and any other) larvae from getting established.
...it's the larvae in soil that can become problematic.... the dunks come as pucks, or sometimes (at OSH) I found Bonide brand granules, which is much easier than crushing up those damn pucks! Those are made for slow release in ponds, etc.,... In any case, to maximize the effectiveness, I found you have to help get the bacteria get a good start; get a 1/2 gal of clean water, no chlorine (let it gas off for a day in using tap), and a couple Tablespoon's worth of stuff, plus a fat 1tsp of molasses and a dash of nutes, and mix it all up,... let it sit for a day or so, to allow the bacteria to come out of dormancy and multiply, then scoop up, bits and all, and water normally.... I find that 1-2 treatments per grow is all you need, and I'm outdoors, where the supply of gnats is unending! At night when they come in for extra light hours, I find few to none around,... 
