Just to make answer short, in my own knowledge. The SPS needs to grow by building their structure with calcium carbonate , so the Calcium you measured is part of the formula. For carbonate, it is obtained from the bicarbonate of the water. Alkalinity is a simpler alternative way to measure bicarbonate (science behind is deep), when Alkalinity is low means carbonate is low for sps structure growth, that's why high KH enhance growth. If your KH is low, you need to use Kalk or KH dosing to increase it. Using Kalk is good as it increases Ca and KH, but got to watch out for PH not to exceed your comfort zone. I am also new, this is what I understand from my kids who studied Chemistry. Lolz Sent from my iPhone using Tapatalk