That looks exactly like the issue I was having, I believe it is a cal-mag deficiency, and what helped me is to raise the humidity in the tent. The low humidity causes the plant to use more water thru the roots than it otherwise would, which can cause deficiencies.I find the flouride keeps the cavities away lol. I always have used city tap water just make sure to sit for 24 hrs and make sure its room temp first. Only thing i see is your temps and humidity is pretty low i find i get nutrient issues if i run to low with a low ph. ....
With a small evaporative humidifier ($40 at rite aid), I am keeping the humidity at 55% and the plants are thriving and growing vigorously. As soon as I let the humidity drop below 40%, I start to have issues again.