White Widow

In soil you don't need to add cal mag with every watering if you are using tap water but you can't go without it if you are using reversed osmosis water . My thaught is that you are using tape water like 90% of us on GD so your water contain cal/mag , you soil have a bit of cal/mag inside or at least some mag ( they are always adding some mag in the soil , must be cheap ) , you have cal/mag inside your nutrients or at least mag so my guess would not be a calcium deficiency but it's not really looking like an excess neither . When I take a close look at some of your tips you can see that some are starting to be yellow or slightly burnt so may be a nutrient excess that created a salt buildup in the soil and a nutrient lockout for the plant. Flush your medium