It means that calcium and alkalinity are most used to deposit calcium carbonate. In that context, if alk is being used, then most likely, so is calcium. The relative rate is about 18-20 ppm calcium for each 2.8 dKH of alk consumed.
My recommendation is to replace alk as needed and use a method that adds that appropriate amount of calcium. A number of different methods add them together that way:
The Many Methods for Supplementing Calcium and Alkalinity - REEFEDITION
There is no aspect of reef aquarium chemistry more important than calcium and alkalinity. Many of my previous articles have described various aspects of these systems in detail. In reading those articles, aquarists will note one pervasive theme: that maintaining appropriate levels of each are...www.reefedition.com
Thanks! I am going to use anhydrous calcium chloride. Online, I found that 1 dKH is equivalent to 18ppm. Based on your response, that would be 50.4 ppm alk to ~19 ppm calcium. From there I worked out that for 0.2 dKH per day, I would need to add 1.43 ppm of calcium per day. Pretty small amount, but doable.