As with fish, there are a range of water parameters that need to be monitored and regulated; the most popular being salinity, pH, nitrate (nitrogen cycle). With corals though there are a number of additional parameters that need to be monitored regularly; kH, Ca, Mg, and PO4 being the most common. There are also a number of other considerations; flow, light, food, pertinent ancillary elements. Most common corals in the aquarium are either considered hard or soft. Bivalves (clams) can be treated much like hard corals in their requirements and usually anemones will also thrive in systems where hard corals thrive.
Hard corals are generally broken down into two categories; large polyp and small polyp. Both types of corals lay down calcium on a hard skeleton via the polyps, forming aragonite. Most other calcareous organisms form calcite, a much harder substance. This means they (corals) require adequate levels of calcium and carbonates to grow. Generally a level above 400 ppm of calcium is desirable and many authors recommend a range of around 480 ppm of calcium for these corals. A hardness of around 10dKH is also recommended and usually maintaining a consistent level of alkalinity, or carbonate hardness, is much more crucial to successfully keeping hard corals. Some corals in particular are extremely sensitive to drops in alkalinity e.g. chalice corals. Alkalinity is usually also depleted much quicker and must be watched more diligently. The third major component is magnesium. Magnesium not only helps keep carbonates in solution, but it also helps bind phosphates which could otherwise bind themselves to carbonates in your coral, your liverock, and your sand; preventing coral growth and creating an algae hot house. Molybdenum and Strontium play an important role in the proper development of reef-building organisms. These elements support healthy calcium deposition for the creation of skeletal material necessary for vibrant growth of corals, tubeworms, clams, and calcareous algae. Iodine/Iodideis essential for the health of many soft corals, such as Xenia and the 4 species in the zoanthidae family, and may also have antibiotic properties in stony coral aquariums, which may also enhance their coloration. There are other important trace elements that are also required and their importance will vary among corals.
The majority of hard corals are also photosynthetic, meaning they will require a certain degree of PAR, or photosynthetically available radiation. Because they are photosynthetic, their major food source comes from the secretion of sugars by the symbiotic algae that live within them, zooxanthellae. Many of these corals, however, can be fed small bits of seafood (shellfish, crustaceans, squid, fish). Typically these corals are LPS corals, and generally the fleshier the coral, the more likely it will require meaty protein rich foods. Dendrophyllia and most of the brain corals (trachyphyllia, acanthaphyllia, cynarina, scolymia) are prime examples. Many of the LPS also corals have long stinging tentacles called 'sweeper tentacles' which serve to cut down on competing corals in the nearby vicinity so you must give them plenty of room in the aquarium.
The hobbyist idea of "soft corals" is quite different than the actual definition. In the aquarium we usually speak of star polyps, zoas, leathers, and cloves, et.c when we speak of soft corals. Their care is typically quite easy and they often thrive in systems populated with fish as they can handle the typically dirtier water of “fish” systems and in addition, pull nutrients from the water column. Soft corals benefit most from iodine/iodide and the largest threat is lower pH levels.
True soft corals, according to the definition, all belong to the subclass Octocorallia. The name "Octocorallia" refers to the fact that each polyp has eight tentacles. This can be confusing since many 'soft corals' are not actually soft. This definition includes such corals as the "Blue Coral", Heliopora coerulea, the Pipe Organ coral Tubipora musica, and the Gorgonians, all of which produce hard shells or skeletons. Another characteristic of true soft corals are the side branches of the polyp tentacle, called 'pinnules', which give the polyps a feathery look. Although pinnules are a sure sign of a soft coral, not all soft corals have them.
To the hobbyist soft corals are those that lack a hard skeleton like stony corals and don't have a tough skin like the leather corals. This is basically what we are listing in this section with the addition of the blue coral since it doesn't fit easily into any of the other categories.
Many soft corals contain zooxanthellae and so need a lot of light. On the other hand many of them, like the carnation corals, grow on the underside of reef ledges or shaded areas and don't require light. They usually must be fed to survive in the aquarium.
Leather corals are a soft coral with a rather tough leathery skin. They frequently contain zooxanthellae and therefore do well in bright light. They require currents to help them slough off a milky slime on their surface which is produced to rid itself of any algae or other irritants that may be present on the skin.
The gorgonians are soft coral colonies that are tree like. They attach themselves to rocks at the base and have a skeleton that is similar to animal horns.
Many gorgonians are photosynthetic and so need light to survive. Others, like the finger corals, grow at great depths or shaded areas and don't require light. They usually must be fed to survive in the aquarium.
The gorgonians all need strong currents to help rid themselves of the waxy film that is secreted to rid themselves of algae. Surge devices or turbulent water flow is best.
Care of Photosynthetic Gorgonians:
Since they are photosynthetic, they need good lighting as well as strong currents. Although most are not accustomed to the very strong lights produced by metal halide, it is said that they will eventually adjust and grow faster because of it. Some can be fed while others cannot. In either case photosynthetic gorgonians don't need to be fed in order to survive.
Care of Non-Photosynthetic Gorgonians:
For those that need to be fed, feed them at least once a week. Feed detritus, brine shrimp, Daphnia, Cyclops, or pulverized flake food, shrimp or clams. Sometimes you can stir up the gravel slightly in order to mix detritus in the water for feeding the detritus eating species. (http://animal-world.com/encyclo/reef/.htm)
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