The grit (granite) is necessary for their digestion. It lets them grind up their food in the gizzard. Without it they cannot properly digest and some birds can get blockages. I would use the supplements at this point rather than trying to do it with dietary things. When they get deficiencies it often takes much larger doses, which you can really only get from supplements, in order to get them reversed. Once reversed, if they are on a well balanced diet, then they can usually maintain their levels normally.