Bro, ive been growing along time, I know my shit. Magnesium shows itself in many different forms, I know from experiance, while you think its just one simple answer.
Its hard to judge whats wrong by 1 little picture, but I see a magnesium deficiency (purple stem, light colored leaves)
It might also have some light burn from folar feeding, and/or burning from a lockout, but its hard to say exactly what it is from 1 picutre of A leaf. But his plant is lacking magnesium, as I can see it with my own eyes.