Learn the requirements and how to identify the main groups of corals!
In the hobby there are many different types of corals you can get for your aquarium, but with thousands upon thousands of species knowing the requirements for each one becomes nearly impossible. Fortunately almost all of the corals we have access to in the hobby can fit into a few large groups with mostly similar requirements across each group. By learning the different groups you can understand new coral's general requirements without knowing exactly what species it is.
For the most part, we can group almost any coral into one of three groups. We have the SPS (Small Polyped Stony) corals, LPS (Large Polyped Stony) corals, and soft corals. These groups are not really taxonomical groups, but rather convenient groups for the hobbyist. Within these groups they are usually divided into the coral's family or genus and eventually species for the specific corals. There are many variations within species as well, similar to variations in dog breeds within Canis familiaris. For simplicity it this article will just cover the large groups which all the corals may be placed in. Just knowing these groups can provide great insight into the placement and requirements of corals and could make the difference between their death and propagation.
SPS corals are most easily identified by their namesake. They are corals which have very small polyps (flowery looking dots covering the coral) on a hard "stony" skeletal base. These corals do not move with currents in the water and very hard to the touch. Their skeletal base is made out of calcium carbonate and grows fairly slowly compared to plants. Depending on thickness of the coral branches it can be easily broken into smaller pieces, which given the right conditions will grow into a new colony. These corals are the reef building corals. As they grow upwards trying to reach the light they will shade out and kill other corals leaving behind their calcium carbonate skeletons. These skeletons eventually trap sediment and form rocks and substrate which other corals can grow from. Live rock is almost always ancient SPS and LPS corals which have died and become rock.
Generally SPS corals will require the most light and flow of the corals. If you do not have very strong lighting like metal halide or many T5 flourescents with individual reflectors you will have a very hard time getting them to grow. They also prefer to have strong flow blowing across them. When determining flow, it's often too much if you can feel the flow underwater, but too little if small particulates in the water are not moving past the coral. Water quality is extremely important to SPS corals and extended paramaters must be tested to keep them. They typically will not do well unless Calcium, Alkalinity, Magnesium and Nitrate are regularly tested.
LPS corals are great for beginners with adequate lighting on their tanks. They are fairly easy to keep and can handle poorer water quality than SPS corals can. They are easily identified for they will be the only corals with large fleshy heads but a hard skeleton underneath. They will often sway with the currents in the water retract when agitated. They may grow a lot of skeleton and branch frequently or you may not see hardly any skeletal growth depending on the genus and species. SPS are more difficult to see feeding, but LPS corals can often be easy to feed. Typically there are a set of a tentacles surrounding the centrally located mouth. These tentacles will often come out when food or nutrients are in the water or at night when they are least likely to be nipped by a fish.
LPS corals require greater lighting than standard fluorescent lights found on fish-only tanks, however they do not require as bright of light as SPS corals. Some species will do great under metal halides or other bright lighting, while other prefer shady spots in extremely well lit tanks. They are not as picky about water quality as SPS in some regards, however since they still have a skeletal base water parameters should still be tested occasionally. Calcium, Alkalinity and Magnesium are important factors to test for since they are required to build the corals skeleton. Nitrates can be significantly higher than with SPS corals and testing is not always as important unless they become excessively high. Nitrates should be under 20ppm for LPS corals.
Soft corals are can be identified by their lack of skeleton. They are often most responsive to water flow and are some of the easiest corals to keep. They come in many different shapes and coloration, however they often have the same requirements as each other.
Soft corals can be kept under low lighting and even standard T8 fluorescent depending on species. They do not require the water to be tested for the extended parameters and will only require basic water maintenance. Nitrates may be higher than for LPS or SPS corals, but still should not be excessively high. They often have a low flow requirement and do not necessarily like having a lot of water flow that a SPS corals would. They are good beginner corals, however they can be difficult to remove later, so thoughtful placement should be used.
Knowing the basic groups of corals will hopefully simplify placing corals in your tank. Knowing the high requirements of SPS corals and low requirements of soft corals will make it easy to know if that pretty coral at the local fish store is right for you. If these groupings are still new to you, go ahead and check out the gallery and try and guess which corals belong to which groups!
There can be some variation in requirements within each group, so don't be afraid to ask the seller what conditions that specific coral does best in!