Brown spots on lower leaves - ph fluctuations?

Hi mate, you're absolutely right. This is due to PH. It really depends on the kind of water you have used. Tap water ions react with nutrients and ph adjusters to raise or lower the ph. I started using deionised water for this reason and have seen great results because of it. Reverse osmosis water works too. You clearly know what you're doing so keep up the green work, good luck and happy growing