Crushed coral is for sale in many pet stores for use in both freshwater or saltwater aquarium. Besides being aesthetically pleasing and long-lasting (in fact, it need never be replaced), crushed coral is right for fish tanks that need a more alkaline pH. Crushed coral is perfect as a substrate on top of an under-gravel filter. If used correctly, crushed coral can be beneficial to certain freshwater and marine aquariums.
Crushed Coral Raises pH
Your tank's ability to maintain a consistent pH is called its "buffering capacity." Crushed coral is known for dramatically increasing the buffering capacity of an aquarium. Crushed coral also adds to the alkalinity of a tank, raising the tank's pH. Therefore, crushed coral is best utilized in a tank that requires a higher, more alkaline pH, such as a freshwater cichlid tanks or a marine tank. Some crushed coral comes mixed with argonite or limestone gravel, further adding to its buffer- and pH-raising properties. Many aquarists will use crushed coral in their filter, sump or refugium rather than in the aquarium itself. This gives the tank the pH-and-buffer raising benefits of crushed coral without the problems of crushed coral. Debris gets caught in crushed coral easily, cleaner crabs and snails have difficulty maneuvering through crushed coral and fish who like to "dig" (using their mouths to suck in and spit out substrate) may choke on larger pieces.
Crushed Coral Houses Beneficial Bacteria
The craggy surface area created by a substrate of crushed coral is an ideal place for beneficial bacteria to flourish. These bacteria will perform the important task of turning harmful ammonia from decaying food and fish waste into less harmful nitrites and nitrates, which are removed from the tank during regular water changes. In a marine tank, crushed coral works best as a thin layer on top of a deep bed of live sand. In a freshwater cichlid tank, a couple of inches of crushed coral makes a decent substrate. Either way, the crushed coral must be vacuumed with a gravel-vac frequently to avoid buildup of fish and food waste.
Crushed Coral is Attractive
Many aquarists simply enjoy the look of crushed coral. It gives a tank a natural, "marine" look, and its white color enhances the colors of fish, corals and plants. The varied, unique look of each piece of crushed coral adds visual interest to the fish tank.