What Fruitbat says is correct.
Calcium carbonate-based shells and rocks will indeed raise the KH
and the pH in your tank, making it more suitable for African cichlids in those tanks with either an unstable KH
, or a pH that is not ideal. In addition to the limestone, you can also use crushed coral which can be found in the substrate department of your LFS
. To use this, you can either put a few tablespoons in a mesh baggie (found in the paint-straining department of Home Depot for about $2) and place it in your filter compartment or mix it directly into your substrate.