I have been fighting a problem with flies. Not white flies, little flies that a plumber buddy called sewer flies. He said with all the rain they have been a real pain in our area and don't feel alone. They have a tiny worm thing that lives in the roots. I tried neem oil soap, on the leaves more that once, like each week like they say but still had them. I was wondering if I would be better off to add it to the res. directly, no soap just some oil. just a little once in a while. It is supposed to screw with there systems. Question is how much in a 20 gal res?? Or will it kill the plants. Thanks for the help