These corals belong to the hard type and are much used in reef aquariums. Writing a text on them in general terms is the only way given the space available.
They are given their common name because, obviously enough, the branches of the coral generally resemble the horns of a stag, though the branches vary considerably. There are other common names such as tabletop coral and cat’s paw coral (some have flat tops as does a table so there are also those that resemble a cat’s paw), but these names do not fit so well overall and are not as widely used. The proper name for the type being looked at here is Acropora. I can immediately find 47 types of Acropora in the references available to me and I’m sure there are many more, one suggests a total of 300 or more. They each have their own proper name under the heading of Acropora, hence the general look only.
As a hard coral they belong in a reef aquarium of course. The aquarium should have powerful lighting such as metal halides, probably best supplemented by, say, two actinic fluorescent tubes. High seawater quality is a must, and the calcium and alkalinity levels need to be monitored. Acropora corals demand calcium and the amount of usage can be surprising in a well stocked reef, so supplementation from commercial sources could be possible in a small system, but it is likely to be too expensive and cumbersome for a medium or large one. In the latter the usual practice is to install a calcium reactor. The calcium level is best kept between 420ppm (parts per million) and 450ppm, though some aquarists elevate this to around 480ppm. Such a high level is not really necessary as long as the suggested lower levels are maintained. Alkalinity in an aquarium is best if it is higher than natural seawater levels. Alkalinity resists the tendency for pH to drop because of acidic action, in other words it acts as a buffer. The best level in an aquarium is between 3.5 meq/L and 4.5 meq/L. 4.0 meq/L seems reasonable. Some fluctuation doesn’t matter provided it is fairly minor. (NB. meq/L can be converted to dKH if desired, multiply by 2.8.)
So the corals need high quality seawater with appropriate calcium content and alkalinity, plus correct lighting. The lighting could have an effect on the coral’s colour; the colour could change from that when purchased. This change is usually because the symbiotic algae, the zooxanthellae, are reacting to the lighting available.
Another requirement is seawater movement. The majority of the corals are used to high seawater movement and this should be duplicated as far as possible. A minimum flow rate suggested by the guideline is twenty times the display aquarium net gallonage per hour, which is double that suggested for a soft coral display. This flow can be provided in the usual ways such as powerheads. It is worth considering the high output types with wide delivery nozzles as opposed to narrow nozzle types, particularly the ones that can be controlled and made to alternate seawater output level. Whatever type is chosen, they need to be positioned so that a jet of seawater is not directed straight at a coral, though this isn’t as important with wide nozzle types.
Not all Acropora corals require high seawater movement, though as said the majority available to the marine aquarist do. One way of generally judging seawater flow requirements is to consider the colouration, if it is bright then it is likely that intense light is required which brings the coral closer to the surface where strong movement prevails. Another indicator is the length and shape of the branches, if they are short and club like they resist high seawater movement better; longer and more slender branches are more likely to break. Generally, if there are any doubts place them in an area of high movement and, as with all introductions to the captive reef, observe to ensure all is well.
Acropora corals could worry the aquarist when they are first placed in the aquarium. Even though the conditions for them are excellent, they could take a while to settle before commencing to grow. Once they have settled however, and provided conditions are maintained, they shouldn’t be a problem.
When placing the corals allow room for growth, if conditions are good they are fast growers, some more than others and this growth can be such that the reef becomes overcrowded fairly quickly. Having said that, the corals are excellent for ‘fragging’, a term used in the marine hobby for culturing additional corals from a mother colony. This practice is excellent for the hobby and for the wild reef: for the hobby ‘fragged’ corals appear to be more resilient than those from the wild, for the wild reef if corals are ‘fragged’ not so many are required to be collected. Therefore if the captive reef does start to become overcrowded ‘fragging’ is a definite action to consider, and it isn’t difficult.
Though it varies, the branches of the corals are fairly fragile and snap easily, more so towards the tips. Therefore care is required when they are handled. Sometimes they can be positioned so that rocks hold them until they establish themselves, but it is probably better to secure them with some aquarium epoxy putty, which sets very quickly.
Acropora come in all sorts of shapes and colours. Most seem to be branched and resemble a terrestrial bush to some extent, with generally uniform branches sprouting out and upwards. These branches could, as said, be thick, short and club like or longer, dividing and slender. Others grow to a flat top shape thus the name ‘table coral’. The pink and blue of some types are, to me anyway, particularly lovely, though the colours vary with many others.
These corals are more demanding than most, but not all, of the soft corals and the aquarist has to be sure that he/she is willing to provide the ‘extras’ required – seawater should be top notch anyway, with very low nitrate and phosphate. The extras are controlled levels of calcium and alkalinity, high seawater movement and sufficiently powerful lighting. A stable environment is required as generally the corals tolerate changes poorly.
The link gives many pictures of the corals. Click on a picture to enlarge.
If the aquarist can provide the environment for the corals, the reward is a colourful and very ‘reefy’ display. A successful hard coral reef display is regarded by some as the pinnacle of achievement.