I used to use Kalk exclusively, but I dont use Kalk anymore as I have a tank that has a very high demand for calcium and alakalinity that can not be maintained through topping off with Kalk alone.
Others have mentioned that I might want to top off with kalk, and then boost Calcium/Alk with two part. But that seems like twice the trouble. I would need to make Kalk and dose two part. Furthermore, I would neve no exactly how much two part to use as I wouldn't know exactly how much Kalk had been added on a daily basis though the Autotopoff without testing everyday. I am able to maintain specific Calcium/Alk ratios with two part, and I adjust the amount of two part that I use monthly.