Calcium & alkalinity
 

I am unclear on the relationship between the two. I am currently testing calcium and pH, but not alkalinity. It's been a few years since chemistry for me, but I thought pH measured alkalinity?? Does adding calcium lower alkalinity?

I believe that Alk and Calcium balance around Mag? I could be wrong but if your Mag is low Alk and Calcium become unbalanced (one usually being high the other low).
Here's something to read.
http://www.advancedaquarist.com/issues/nov2002/chem.htm

Chris has given you a good start with that article. Just follow the links therein or google "Randy Holmes-Farley" then get a comfy chair and a cup of coffee.