Everyone with hard corals tests Calcium, I've been for years, but am starting to think it is a waste of time and money. I add Calcium and Alkalinity in balanced parts, test Alkalinity, and a small variation in Alkalinity can be a big deal. I've seen no difference between 350ppm and 420ppm Calcium. On the very rare occasion Alkalinity is fine and Calcium is high or low, I find myself assuming it is probably a problem with my test kit and not my water. So what advantage does testing Calcium have?