brown spots between the veins

[Frank]

Hey Ron,
Thanks for all the help so far. I am having a problem with my plants developing brown spots between the veins of the leaves. It started on some of the bottom fan leaves, now it's becoming more and more uniform. I am guessing a (Mg) deficiency but it could also be iron. Please answer asap I lost 8 of 10 plants to an earlier over fertilization episode. She is my last hope. I think the other one is a he and will have to be terminated.
Thanks bro,
Frank