Watering with biobizz calmag everytime or just once a week and just observe? do i cal-mag in end of flower ??

Yes, you can use your tap water and add Cal-Mag with each watering. Continue adding Cal-Mag until your plant is finished in the next week or two.

@ 7.4 PH you are locked out and adding nutes wil do nothing.

"also note" not "not"

is this a soilless medium? then you need to add a proper amount of calcium as well as everything else into every irrigation. I see you are in soil, so how much you add depends on what is in the soil. Lots of garden lime for instance can add a good bit of Ca, as well as gypsum. Both commonly found in soil, but taht stuff runs out eventually and you have to supplement more each week. This takes trial and error as well as using familiar products for consistency. also, not that calcium and magnesium are 2 unique nutrient needs, not 1. Each has their own symptoms. Mg is a weird one. It takes 30-35+ days from the start of a deficiency to see the symptoms on the leaves. So when you see interveinal chlorosis with some spots, you know the Mg issue started a month or more ago. you need more a month ago, in other words. That's an important tidbit to avoid it the next time. you know to supplement more 30+ days prior to this point in your next grow to avoid it, if you use all the same soil and fertilizer products and there's some consistency from that brand. Calcium symptoms are spots without any chlorosis. This will show up when the deficiency starts. I don't see either of these issues in your canopy. The coloration i see is from genetics or colder temps at night (at or under 68F or so). The pale leaves on the ones plant are only at the top. Was that plant by itself before this point? maybe with more intense light or the light a bit closer at same power? If so, might be light intensity being slightly too high at the top causing some cannibalization of those leaves. Diagnosing from symptoms is always a crapshoot. If you have more info, use it to make some common sense decisions... Take notes.. try to do better next time until it gets easy seed-to-harvest.

Use tap water, just leave out for 24 hrs to de-chlorinate and then pH down to 6 then add calmag. Or if feeding, if using pH perfect nutes just de-chlorinated tap water add nutes they self pH to 6.2. If not keep pH between 5.8 and 6.2 👌💚 happy growing buddy 🙌