Do realize that corals uptake Ca and alk at roughly the same rate. I believe the math is 18 ppm alk (1.0 dkh) and 7 ppm Ca, or thereabouts. In other words, a 2 dkh drop would also result in a Ca drop of around 14 ppm. Note: this assumes the reduction is due to coral uptake. Obviously, if you did a large w/c with make-up water that was very high in one and very low in the other, that could also make a difference. That said, I use IO mixed to around 1.026, and I manage my Ca/alk only with balanced Ca/alk additions (kalk and Ca reactor). Any increases/decreases I see in alk affects Ca in a predictable manner and vice versa. Therefore, I know that my use of IO is not causing imbalances of Ca/alk. Also, Ca and alk can get out-of-whack if you let Mg get too low. To be safe and not have to worry about your test kit being extremely accurate, you should keep your Mg at a minimum of 1300 ppm.
