Once you begin monitoring calcium and alkalinity on a regular basis, and if you notice that levels of those ions decline more than can be compensated with a monthly water change, then you'll need to add Ca and bicarbonate (alkalinity) supplements.
A very easy and quite inexpensive way is to make your own solutions of calcium chloride, sodium carbonate, and magnesium chloride/magnesium sulfate.
Recipes for these three solutions are found at the following link:
http://reefkeeping.com/issues/2006-02/rhf/index.php
I bought 50 lbs of Dowflakes last year so I can provide you with a couple years supply for just a few bucks. You can get the sodium carbonate at any food store as baking soda. The magnesium chloride is sold as Magflakes, and that can be ordered from either of these two sources:
http://www.buckeyefieldsupply.com/showproducts.asp?Sub=124&showspecials=124
http://www.twopartsolution.com/
Your hard coral (SPS and LPS) or your encrusting coralline algae extract calcium and carbonate ions from the water to grow their calcium carbonate structures. Some people, myself included, add calcium and alkalinity in a single solution of kalkwasser (aka limewater aka calcium hydroxide). While that doesn't add carbon atoms into the water (the backbone of the carbonate ion), the hydroxide ion of the kalkwasser reacts with carbon dioxide from the atmosphere to create carbonate ions in the water.
You can make kalkwasser with pickling lime, which can be bought at Kroger's (they carry the Mrs. Wages Brand), found in the canning section. I add kalkwasser to compensate for water lost from my tank due to evaporation, but my coral consume more calcium than that, so I have to add the two part homemade solutions as well.
Jay
