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