The claw/canoe leaves tell me you are using too much fertilizer in total. The PH fluctuations may be part of the problem. I would try use less total ppm and control the PH a little closer but I am no expert.
I have never seen powdery mildew and fungus gnats in a hydro setup, only soil. Do you have both together by any chance?

Whatever you do, don't chop em. Get it figured out.