That doesn't really look like a calmag problem to me but it doesn't hurt to add calmag if you haven't been. It honestly looks kinda like nute burn.. 1.8 EC is a bit high for plants this age. I've been feeding around 1.3ec and my plants have been loving it. Granted I just grow in coco, not dwc, so that may be something that is a bit different between the two. I also see your ph is at a 6 which is okay but I've personally seen better results in my coco keeping the ph around 5.6-5.8. Either way despite the crappy looking bottom leaf the rest of the plant looks perfect! So it may just be an odd thing that happens and it may not really affect the grow as that'sIve a lower leaf that you would prune anyways.