There should be enough Mg in tap water to not have a deficiency. Conversely, if you use reverse osmosis, you should add some Cal/Mg.

You might also be root bound and need repotting.
WashougalWonder Reviewed by WashougalWonder on . Possible Magnesium Deficiency? My 2-3 month old clones are experiencing yellowing of new growth. The yellowing is localized on the perimeter of the leaf, leaving an arrowhead-shaped darker, healthier green in the center of the leaf. Researching google, I diagnosed this as a magnesium deficiency (whether I was right or wrong) and followed the directions posted under the sticky "epsom salts = not miracle cure)," spraying the leaves (yesterday and today). The condition seems to have become worse since applying epsom salts. My Rating: 5