Hardening your water

Hardening Your Water (Raising GH and/or KH)
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 25 gallons 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 25 gallons raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.