This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.
The following measurements are approximate; use a test kit to verify
you've achieved the intended results. Note that if your water is
extremely soft to begin with (1 degree KH or less), you may get a
drastic change in pH as the buffer is added.
To raise both GH and KH simultaneously, add calcium carbonate (CaCO3).
1/2 teaspoon per 100 liters of water will increase both the KH and GH
by about 1-2 dH. Alternatively, add some sea shells, coral, limestone,
marble chips, etc. to your filter.
To raise the KH without raising the GH, add sodium bicarbonate
(NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters
raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards
an equilibrium value of 8.2.
 
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