What I usually do if I'm not too sure is give a good flush and then give it a little bit of a product called Revive (from AN) in a foliar spray. It contains all the micro and macro nutrients that the plants are most likely to be deficient in (iron, zinc, magnesium, calcium, nitrogen and other crucial nutrients).
BUT, if it is a deficiency caused by over feeding than I'm not sure this is the best route... maybe just give a good flush and wait...