FYI, Altolamprologus calvus is not a brackish water fish but a Tanganyikan cichlid. While both brackish and rift lake environments have hard alkaline water, brackish environments are much more saline. Crushed coral is typically used as a substrate for both rift lake and brackish aquaria to help buffer pH and keep it alkaline. However, crushed coral does not make the water brackish, sea salt does. If you use crushed coral as your substrate, as opposed to merely putting a small bag in your tank or filter, there's usually no need to change it periodically in light of its substantial buffering capacity.