Just shut down. Had started tank about a year ago. About 3 months after I set it up the sand bed was very hard. It was so hard I had trouble breaking it up. At that time I was not dosing anything. I started out with about 4 inches deep of fine grain aragonite. It kept getting hard after it was broken up where I could get at it. For over a year I could not keep my alk up. Any ideas what would cause the sand to calcify?