Calcium Carbonate Buffer

Sack of Lime (CaCO3)

You can use drug store calcium supplements (such as Caltrate tablets) for buffering in a pinch, but calcium carbonate (CaCO3) at the garden center is cheaper. For example, this 5-pound (2 kg) bag of "Limestone Flour" from Walt's Organic Fertilizer Company cost about $5.00. As you can see from the photo, it's gray, not chalky white, and it's a little coarser than the pharmaceutical grade, but it dissolves quickly and works fine. For my buffering needs, I add 1/2 teaspoon (3 grams) at every 20-gallon (76 liter) water change by just swirling it into the water with my hand. By the next morning it has all dissolved.

Powdered Limestone Analysis
Calcium Carbonate (CaCO3) 97%
Calcium 38.8%
Magnesium Carbonate (MgCO3), max. 2.0%

Dolomite lime has roughly equal amounts of calcium carbonate and magnesium carbonate, so it's a good source of magnesium in addition to calcium.

Make sure that your not getting calcium oxide (CaO, also known as quicklime), which is a caustic chemical that is used to make cement.