That looks more like a mirconute deficiency. It could be magnesium but that word gets thrown around so much it's kind of hard to tell. You have brown spotting but when the leaves were more green, did they develop a golden yellow spotting at first? If so, it's probably magnesium def. At any rate, you can solve micronute deficiency with a well-rounded fertilizer. If it's just magnesium def, you can beat it with one half teaspoon of epsom salt in a gallon of water, which you can buy at a drug store. Best thing is to use a more complete fert. I take it you're growing in soil. You'd go to your local grow shop and reading the back of the containers, look for a fert that has a bunch of nutes to it, like boron, sulphur, mag, etc. Dynagrow does and I think Supernatural does. You'd get the Terra version if you're in soil. If you've been ferting, don't give much, just enough to try to fight this problem. Follow the directions on the container.