Stony Corals, also known as Scleractinians or hard corals are the founders of any reef and get there name from there ability to secrete a calcium carbonate exoskeleton. There is 3,500 species documented and most of these contain zooxanthellae from which the stony corals gain most there nutrients.
Each group of stony corals require slightly different conditions, for example small polyp stony corals (SPS) prefer strong aquarium lighting and water currents compared to large polyp corals (LPS). It understood that Large polyp corals are easier than smaller polyp corals but actually both require good,stable water conditions and maintenance to thrive.
The most common forms include conical and horn-shaped scleractinians. In a colonial Scleractinia, the repeated asexual division by the polyps causes the corallites to be interconnected, thus forming the colonies. There are also cases in which the adjacent colonies of the same species form a single colony by fusing.
Stony corals are colonies of individual animals. Each animal, called a polyp, lives in its own calcium carbonate skeleton called a corallite. Each successive generation of polyps living on the skeleton build up of the previous generations. This reef building abilty gives hard corals the label hermatypic.
All hard corals are to a greater or lesser extent predatory, catching food with their tentacles that are lined with stinging cells. However the majority of hard corals have also formed a symbiotic relationship (beneficial to both parties) with zooxanthellae, single-celled algae that live within their tissues. The algae carry out photosynthesis sharing the sugars and oxygen produced with the coral polyp. The coral returns the favour by providing protection, useful waste products like carbon dioxide and nitate plus simple minerals.
These corals with a symbiotic relationship make up the majority of hard corals and live close to the surface, where there is plenty of sunlight. The extra source of food enables them to grow faster, than hard corals that don't have a symbiotic relationship (called ahermatypes) arguably making them the main reef builders of our oceans. This is why the term hermatypic is often used exclusively to describe hard corals containing zooxanthellae.
It should be noted that many other animals also produce calcium carbonate skeletons, including some soft corals.
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