Yellow Leaves

Looks like you had nitrogen deficiency, then you over corrected and now you have nitrogen toxicity. If you're in flower, it could be a potassium deficiency. I think your best bet is to flush and reset your nutes. Remember the manufacturer's recommendations often are too hot and you need to hook half strength. Also remember that affected leaves will not go back to normal, so your goal is to stop the spread.

Judging from the photo I'd say that it's a lockout of nitrogen which is causing this. Hopefully, that answered your question and feel free to ask for advice anytime, and remember ..."Happy Growing" and "Free your mind one puff at a time"