Low cal mag?

Soooo, I would ask you where are you in flower. Plants don’t need Calcium in flowering. They do need magnesium for about 300 separate processes tho. I would say add 1/2 tsp Epsom salt with 1/2 TBSP molasses ( sulpher free) in 1 liter of water. This would also rule out a few other things without using nutrients that could block out something else. It would rule out or rule in potassium which also causes yellowing of the leaves. Magnesium will also have spots along with the yellowing btw, magnesium deficiency isn’t just yellow alone.