Question. How are they killing your plants? Are your plants really young? I used this stuff called Gnatrol. Its a bacteria in this powder that you stir into your water. I noticed a difference in about 2 weeks. I used 2-3 tbsp per gal every time i fed. You can add your nutes and everything. You can use it all the way up to harvest. The larvae eat the bacteria and it kills em. There is really nothing you can do about the adults, but wait till they die. They only live about a week. I suggest getting a lot of those sticky yellow fly traps and use that gnatrol.

It sucks I know, ive been there but it will eventually get better if you do this stuff. The gnats shouldnt be harming your plants though unless theyre real young. i know the larvae can damage the roots but if your girls are established maybe something else is killin em. good luck!