Do not over-acidify them. Get the ingoing water within range, and try to keep it there. My wellwater is 7.9, and I use 1/4 tsp of phDown (phosphoric acid) which brings me down to 6.8ish. One gallon of the stuff lasts me over a year, and my ladies love the stuff.
Your yellow leaves won't color-up, but it will slow the progression of yellowing into the new growth. :thumbsup:

To me, gnats are no big deal. But they will never go away without action. But if it's something more damaging than gnats, I'd make sure they didn't live to tell about it. If you do fog again, open your potting soil bag to fog the bugs-n-eggs in there, too. :jointsmile:

Are you in pots, or in the ground? (just curious)