Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Cuttlebone, coral sand, oystershell grit, limestone sand and similar materials will decompose slowly, adding minerals to the water and raising the pH. While each of these are slightly different, mostly they are calcium carbonate. Some are a blend of Calcium carbonate and magnesium carbonate.
They dissolve slowly.
When I have needed to keep the GH (or KH) up in a tank I make the new water for water changes to the specs I want, adding Seachem Equilibrium for GH and baking soda for KH. In the filter or in the tank I have at least one of the items listed in my first sentence, often 2 of them. They are fine for stabilizing the situation, but do not dissolve so fast that I would want to do a water change with softer water and depend on them to make the water harder.