Adjusting one level effects the others.
A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH
http://reefkeeping.com/issues/2006-06/rhf/index.php
I personally don't bother testing the water chemisty on my 20gal nano. Small tanks are too prone to rapid change so its not worth the hassle in my opinion. So I stock lightly, do large weekly water changes and just observe.