Help yet look stressed

I actually believe it is a Potassium (K) deficiency - leaf tips and margins begin to yellow and then to suffer chlorosis, appears more severe on the bottom of the plant and spreads to the top (you can also look up pictures online "Cannabis Potassium deficiency). Treat it by increasing or adding a concentrated Potassium fertilizer (usually the "Bloom" or "Grow" fertilizer, high K in the N-P-K ratios). Increase by about 20-30% or add about 1/4 of recommended dosage of the correct fertilizer. As for the fungus gnats @The8thChevron mentioned most of the ways to deal with them but also remember to remove any dead/dry/decomposing leaves or any other plant material as soon as possible because that's what mostly attracts fungus gnats, also try adding a few yellow stickers to assess the severity of the gnat situation (personally I'd avoid washing the buds post harvest unless you have noticeable amount of dead gnats on your buds). Hope this helps! Good luck and happy growing :)

And, sorry, the burn is toxicity. Looks like you're feeding well enough, but you've probably built up a lot of residual nutrients in your soil. Looks like she's still getting way more N than she can handle at this stage in flower.

Gnats aren't too bad. It's usually a sign that you're watering too often. Wait until the top inch of soil is dry before watering. The gnats will die off within a week without constant access to water. If it's really bad, it may take a few, but they shouldn't do much damage to your girls. Look up bud washing and the 3 bucket method so you can get rid of their corpses and feces when you harvest. Good luck!