Mosquito bits killed every gnat before they even started for me, i don't even use it anymore as i just rewash my promix and keep growing but i still have a big bag ready if the decide to move in.
I sprinkle it on the top soil and water it in and it seems to work best. i have tried mixing it in a spray bottle too and squirting it on the top soil i have also tried mixing it on my res. i think they all work but i like the sprinkle on the top method as it is the easiest and works great