6.8 is not bad. I also use white vinegar, but remember to agitate the water for a minute or so to oxygenate and mix well BEFORE you take your pH reading. Remember also that calcium tends to buffer the water towards a higher pH, and that buffering means that it will resist pH shift.
If you want to use an organic pesticide, go with tobacco tea. F'ing works wonders, kicks the living shit outta neem oil (what a waste that was), and you can smoke your buds safely after use.
10 grams loose tobacco (I use American Spirit or Bali Shag) to 1 liter very hot, not boiling, water. Steep overnight, strain. Add a drop of non-antibacterial/antimicrobial soap (those soaps are the devil), shake it up, spray, ensuring you hit all surfaces as well as the soil.
Oh yes, if you're using fertilizers and still having problems with deficiencies, look into starting them up on something that has Mycorrhizal fungi. Google it, they are amazing.