Cal-Mag provides calcium and magnesium which are considered secondary nutrients (some even consider them primary nutrients so far as cannabis is concerned) as they are needed in more abundant amounts than are boron, iron, zinc, manganese, et al which are the trace or micro-nutrients.
If you're using tap water you really don't need to use any Cal-Mag because it is likely that a sufficient level of dissolved minerals such as Ca and Mg are present already.
I don't really want to get into the whole "soil is a buffer" thing right now. But first of all, many potting soils are not true soils at all and secondly pH does matter in soil but we perceive that it matters much less in healthy, inoculated and properly amended soil (with lime for instance, which is the buffer).
Too much Ca locks out Mg, as well as Fe and K while too much Mg locks out Ca and other trace elements. I fail to see any signs of Ca or Mg def/lockout in those photos, symptoms would appear in the middle of the plant. However, Mg does begin to lock out at a pH below 6.5 (as does molybdenum, but MO is mobile). Therefore, I reassert that you go out and find yourself a good micronute formula and foliar feed with it pronto. You might also try watering in some dolomite limestone for the hell of it; to raise the pH up a bit more and serve as a preventative substitute for the Cal-Mag.
The only other thing I can ask you is what size containers you are growing in? Are you sure that your plants aren't getting root bound?
