BUGS! Non-Toxic. Organically Kill GNATS! Diatomaceous Earth + Hydrogen Peroxide 12% Food-grade

There's no certain time to treat for gnats, you just do it if the need arises. The best practice is to avoid them in the first place by not promoting conditions they thrive in or use inorganic substrates/hydro. I've only had to deal with one infestation that was bad enough to worry about treating them and DE did the trick just fine. If you get food grade it should be safe for your plants.

The best way to treat as @AsNoriu said is to use predator nematodes, you will break the cicle of pupas and you will get an almost clean fungus gnats eviroment.

by the look of it your already treating them and going in the right direction. depending on the knat you can get predator insects too :) best of luck buddy sure it will get sorted :)

I alway used nematodes. Relatively fast, 4sure clean way to get rid. Drogonfli co uk I bought there, i saw on amazon too. Find alternative.