That does resemble sulphur deficiency, but I'm not entirely sure about that. Find an all-around fertilizer, one that has boron, sulphur, mg, mn, etc., and give that plant a dose as stated on the container's directions. Giving magnesium won't help with a sulphur def. You've got to give sulphur if that's what's missing. Many hydroponic ferts will have everything a plant needs. Supernatural comes to mind. Ask for this kind of thing at your grow shop.