I lost!

I don't see, from your diary, that you are even feeding this lady so I'm wondering where you're getting the ppm you mention in your comment for the last day - but then, that ppm does not equate to an EC of 0.4 you report so I'm really confused.. Are you growing in living soil or an amended soil of some sort? I agree that it looks like you're JUST starting a calcium/magnesium deficiency but I'm not convinced simply adding calmag will help. It may be you just need to start feeding them a decent cannabis nute with a good grow NPK. They will normally have enough calcium and magnesium in them so this problem doesn't arise. Yes, there are a lot of people who get those deficiencies but the deficiencies can come from several different sources - a pH that's out of whack (yours is fine), a nitrogen toxicity that causes a lockout (I don't see that indicated here)... So yes, you can just add some calmag (only 1tsp/gallon once a week) or you can give her a well-balanced grow nute and prevent OTHER problems down the line. Good luck - and Merry Christmas!