So I have started getting this rust colored spotting on my leaves 5 1/2 weeks into flower. Nirvana feminized ww in dwc using advanced nutrients con. Bloom, b52, big bud, cal mag, bud candy and hydroguard. Ph to 5.6 and rises to 6.2 over 24hrs for the first few days after rez change and then settles at 5.9.
So a kind of hydro hybrid? I use lava rocks in my aquaponic system, and it tends to stabilize at a pH of 6.4. Of course, it's outside under the sun. I have seen signs of -Ca in it, so I dose with a product called CaFeMg from Sensational Solutions. However, being Ca, and being immobile, it's one of those things that needs addressing from the start.
My suggestion is to try to keep pH shifted towards hydro numbers, and use a Ca-only product (not cal-mag!) as both a root drench and foliar application. You're getting close to end of flower, I would also suggest stopping any additional Ca about two weeks prior to planned harvest.