Raather than guess at a deficiency based on such limited information, why don't you give us the tank size, light type and wattage, substrate, fertilizing method used, CO2 method, if any, used, etc? That makes it a whole lot easier to guess at what could be deficient.
Am in full agreement with hoppy. It might also be nice if we could get a photo or two. Deficiency is very complicated because you might be suffering several kinds and the effects vary sometimes with individual species. Lets get plenty of data so we can provide an accurate assessment.