Classic calcium deficiency. Rust spots around the veins of the newest growth. Did you add any lime to your mix before planting in? I find that promix tends to only put enough lime in the mix to last a short amount of time (enough to put on the ingredient list basically). I often found my lime dissolved about halfway through flower due to the acidic waterings of 6-6.5 pH (dissolving it slowly over time). Once its gone, the Ca bank of your soil is gone, and so is soil pH buffering ingredient. once i started adding 1/4c to every 5 gals of promix, never had that issue again.
i would try topdressing and scratching in some dolomite lime or oyster shell flour. since you're ph'ing your water to the acidic side, it will dissolve with every watering. so maybe 1/4cup ish will get you through to the end. You could maybe get away with a couple TBSP even.
and FWIW, it's pretty tough to give them excess Ca that would affect plant growth. Native soils are chuck full of Ca, and they evolved with that. That is the reason why Ca def affects the newest growth first, they didn't need to adapt to move Ca from old tissue to new tissue, because the soil usually has an abundance. excess Ca would probably cause other issues with your media before it affected the plant growth from excess. For example, keeping pH too high, and locking out trace minerals/micros like Fe, Zn, etc.
and i agree with an above poster... maybe a little Mg def too (in the lowest growth in the one lower shot). so i would do dolomite lime, which has Ca and Mg, and will dissolve with each acidic watering, releasing ions in the process for the plant.
