While saltwater corals are capable of living individually, the types of coral for reef tanks tend to grow in colonies to create a colorful world of intricate beauty, just as they do in the wild. The beauty of coral reefs is due to the interesting shapes and dazzling colors of the coral. Coral organisms tend to live in tropical waters close to the surface to allow the rays of the sun to interact with the algae they host. These algae, called zooxanthellae, provide the coral with nutrients as a byproduct of their photosynthesis giving the coral their vivid coloring. Coral polyps are related to jellyfish and have soft bodies that are attached to the calicle, or hard base. The base is the actual structure of the coral reef and is like a skeleton. Polyps attach to the sea floor and divide into identical clones. As the calicles come together and connect they become a single organism called a colony. The colonies can live for centuries and continue to grow and expand. As the colonies grow, they create saltwater coral reefs by joining together to form the large structures we see today.
Saltwater corals are living organisms that come in many sizes, shapes and colors. However, some of the basic biology is true for the majority of coral. For example, each polyp has a pharynx that both takes in nutrition and expels waste. The most common way to separate types of corals for reef tanks is by dividing them into soft coral (octocoral and gorgonians) and hard coral (stony coral). Once separated into soft or hard coral further division is possible.
Stony Coral vs. Soft Coral
Live soft coral colonies have a stem much like a tree trunk that attaches to a hard surface at the base. The stem is attached to branches that differ in shape from coral type to coral type. Their skeleton is made up of a protein, gorgonin, that is flexible. Soft saltwater corals are an excellent place for a beginner aquarist to start with as they are hardier than the stony coral. Soft coral such as cabbage leather coral, green star polyps, and daisy polyps are able to survive and even thrive in a wider range of water conditions and are more forgiving of any minor learning errors on the part of the aquarist.
Stony coral are a bit more complex to care for than soft coral and require more effort on the part of the aquarist to maintain. These types of saltwater corals for reef tanks are further divided into small polyp stony coral (SPS) and large polyp stony coral (LPS). This terminology allows aquarist to describe the coral based on the size of the polyps. The stony coral are what create the foundation of coral reefs because they produce a skeleton made out of calcium carbonate. Their skeletons grow in plated and branch shapes. Small stony coral are beautiful and can add vivid color to an aquarium. They generally do best with strong currents and bright light. Although they propagate by breaking off pieces they are fragile and are considered more difficult to care for than large stony coral and soft coral. As a result, they are not recommended for beginners. Some examples of small stony coral include: Acropora, Heliopora, Porites, and Stylophora.
Large stony coral or LPS are diverse types of corals for reef tanks, with some that are very easy to care for and others that are extremely difficult to maintain. They are usually easier than small polyp stony (SPS) to maintain in an aquarium. Some can be fast growing with eye-catching tentacles. In order to grow and thrive LPS need sufficient levels of calcium as well as iodine and strontium. They get most of their nutrients from zooxanthellae but some species can benefit from being fed to supplement their diet. These saltwater coral prefer less flow than SPS and need less light because they have more exposed fleshy tissue. Large polyp stony coral have tentacles that sweep the area around them so they need room to move. If other coral are too close to them they will come into contact with the tentacles. Some great LPS to add to a tank that are fairly easy to keep include: Turbinaria, Plerergyra (Bubble Coral), and Caulastrea (Candycane Coral).
Some aquarists prefer to add saltwater coral into a tank with other marine life to add visual impact. Other aquarists choose types of corals for reef tanks to create an aquarium with the entire focus on the beauty and diversity of coral. In both cases it is wise to select the coral that best fits the experience and space that will maximize the enjoyment of the aquarium.