Looks to me like your fan leaves are already turning yellow. I would continue to feed a very light nutrient solution till about 3 days before harvest and 1 day before you actually cut her run straight water through. As far as the fungus gnat problem goes I'm sure there are threads about how to treat them safely. I haven't had them myself so I can't tell you from any experience of my own.
This late in flower, battle the gnats with yellow sticky traps. Instead of hanging them lay them on your soil, adults get stuck, no more breeders, eventually they gone.
I cant find the yellow sticky traps anywhere ive looked. Ive also put de on the top of the soil ive sprayed with neem oil like i said everything was just wrong