Now that leaf is definitely salts buildup.
Checked out your 1st grow and other than the one WW didn't see too much in the way of salts buildup. Nice grow!
If you are using real dirt with dolomite etc in it you shouldn't even need additional CalMag. I've always used only RO or distilled water and never even had CalMag until a few years back and never saw ant low Ca problems even in DWC where the only Ca they got was from the AN nutes I was using. Did see low Mg and treated that with Epsom salts and not much of that.
I see so many posts where people chime in with , Add CalMag and all your problems will magically go away when I can see there are other issues than CalMag. It's not the magic bullet some people think it is and overuse will just make things worse if there are problems.
With DWC I only change nutes once right after the stretch then go with Lucas formula to lower N levels and use Big Bud to boost flower and resin production from a week before I flip to flower. I've done around 50 tubs and a couple hundred plants in DWC and lots more in soil/soilless pots too. I'm shit as a dirt farmer but we all have our niches and mine seems to be DWC.
Tap water, especially hard water, will leave it's minerals in the soil after the plant uses the water so every time you water it builds up in the soil like the scale in a kettle does. Doing the 20% runoff each watering helps a bit but not enough if the water is hard. Town water here is around 360ppm depending on season and has a host of other minerals that can mess with your plants. At least they use chlorine instead of chloramine and stopped adding fluoride around 2006. We buy RO water for drinking as we get our water from a dugout on my property and it's like town water minus the chlorine but add in runoff farmer pollution. Got frogs and little fish in there so it must be fairly good water. My dog lived to almost 17 and he drank it every day of his life.
I found a great spot to download
FREE POT BOOKS. I downloaded a grow bible first and got lots more. Books look great and complete like the real ones I have here. No web site but just a page of links. Just right click on what you want and then "Save Link As" to download so they don't open first as some are 50+ megs. They got lots. Enjoy.