Calcium ions are calcium atoms that have a +2 charge . This means that they are looking for a date with ions that have a negative charge. The compounds used as buffering agent in your tank supply these ions and are your hydroxide ions ( OH)-.
Think about the hydroxide ions as best friends who never separate while on a date. When they are all hanging out in the tank together, its like a massive pick-up seen at a local club. The calcium ion has such attitude ( the +2) charge, that he thinks he can hook up with two girls at once ( OH -). He will hook up with 2 of these at once since +2 and -2 = 0 charge.
However, if there are more calcium ions than there are available OH- ions, then not all of the calciums get to hook up and are left looking for other dates. So the buffering capacity of the OH- ions disappears as the OH- ions become linked with the calcium ions .
Calcium ions will also play this dating game with carbonate ions . You need to be careful as to which compound you use to introduce the calcium ions. If calcium comes in with chlorine as calcium chloride ( a very common calcium additive sold at the LFS
), when the calcium decides to hook up with the OH or carbonate ions, Chlorine is left behind dateless and very unhappy. Increased chlorine ions are not particulary a good thing to have either.