If your alkalinity is in the 20's and you have a "low" total hardness, chances are, your alkalinity is coming from less desirable or beneficial dissolved salts, such as Sodium Bicarbonate. You need Calcium and Magnesium as the predominate salts for the best overall production of fish.

Here is a good explanation..... http://www2.ca.uky.edu/wkrec/Hardness.htm