Most people who get a saltwater aquarium want corals. If you haven’t worked with corals, some aspects of their care may be very surprising. Most are much easier than most people expect. They can be surprisingly hardy, they actually eat, light is even more important than you probably thought, and they can be problematically aggressive.
Basic Introduction
Corals are animals. They are polyps. Although anemones aren’t corals, the two are related and the basic anatomy is very similar. They have a base, a main body, tentacles, and a common opening for feeding, releasing waste, and reproduction.
I will try to lean more toward terms used in the hobby rather than strictly scientific terms because the hobby terms are what we are all exposed to and that is the terminology we need to understand. Most coral names are actually the genus name. For example, Montipora isn’t one type of coral, it is a genus of corals. Since identification at a species level is so difficult at a hobbyist level for most of the corals we will ever keep, the standard is to use the genus name in addition to common names that describe color and pattern (for example, sunset Montipora is a Montipora with an orange base and green polyps). However, there are some corals that go by widely accepted common names, such as mushroom corals. Be aware, some names for colors and patterns get very creative (and aren’t official in any way). Hearing someone start rattling off a bunch of zoanthid names may confuse you as to whether you are buying corals or marijuana.
There are three main types of corals: soft corals, LPS (large polyp stony corals), and SPS (small polyp stony corals). The soft corals do not produce any type of skeleton at all. The LPS and SPS do produce a calcium based skeleton. These terms do not correspond to specific genera or families of corals, these terms are strictly hobbyist terms and all three groups contain many types of corals, some more closely related than others.
If you are interested in an exceptionally well-written book that provides a very in-depth and thorough understanding of corals, Corals by Eric Borneman is the best book I have ever read on the subject and is effectively a textbook on corals.
Color
Coral colors are probably one of the biggest factors that attract people to saltwater and aquariums in general. Although some are harder to find than others, corals are available in every color in the rainbow, and many corals contain unbelievable combinations.
Coral color depends mostly on lighting. People tan brown as a defense against UV light. Corals do the same, only they don’t tan brown, they have pigments of every color. If they aren’t provided with strong lighting, especially in the blue end of the spectrum which includes actinic, they simply won’t have the amazing colors they should. When corals lose color because they are no longer provided with light as intense as they should have, this is called browning out. You may not notice it until you go back to the fish store and see corals that are much brighter than yours. This is a big indicator that your lights are insufficient. Another is corals that are reaching. If you see most corals they spread out, but when they don’t get enough light they will actually reach higher and spread wider in an effort to get as much light as they can. The main body or stalk of the coral polyp will be much longer than normal.
Any time you increase lighting for a coral, do so slowly. Don’t simply move a new addition to the top of your rocks directly under the light. This can be way too much light, way too fast. Again, think of it as tanning in humans. You can’t live in Canada and then in January fly to Tahiti and sunbathe for a week. You have to step up slowly, the pigment needs time to develop. Some corals will simply not open as much if there is too much light, but not all. so don’t think they are fine as long as they open. Clams will actually try to jump off the rock to get to a lower spot if the light is too intense. No matter what, if the problem isn’t corrected then the coral will probably die. Start corals low and move them up a little at a time. It is okay to give them less light than they are used to and slowly move them up, they can handle that. When in doubt, keep them lower.
It is also important to update your bulbs slowly, not all at once. T5HO fluorescent bulbs should be replaced every 6 months, but don’t replace all the bulbs on your fixture at once. Do one every couple of weeks. Write down which ones you have done and when. This way you know which to do next and when each should be replaced again in another six months. I have a six-bulb T5HO fixture and have an email reminder to replace one bulb per month. This way, each bulb is in use for exactly six months, but only one is ever replaced at a single time.
Even severely neglected glass tops being cleaned well can increase light enough to stress some corals. The salt creep and other deposits on the glass can block enough light that when they are cleaned, the tank is visibly brighter and the corals feel it. Keep your glass tops clean all the time!
For a more in-depth understanding of lighting, please read the Lighting article.
Zooxanthellae: Photosynthetic Algae
Zooxanthellae are the algae that live inside the corals. The corals themselves are not photosynthetic, but the algae that live inside of them are. In a reef ecosystem, the water has almost no nutrients, so few that algae can’t live. If there are enough nutrients for algae then the algae can quickly take over and smother corals. This is actually one of the causes of reef destruction in the wild. Algae living inside corals and other photosynthetic reef organisms get their nutrients from the organisms that they live inside of. What would otherwise be waste that is simply excreted into the water is food for the algae instead. The benefit to the corals is energy. The algae inside the corals produce a lot more energy than they need. They don’t have a way of storing this energy, so they simply dump it. Since they are inside the corals, the energy is simply taken up by the corals. This is so important that photosynthetic corals quickly die if they lose their algae, even species that feed heavily on actual foods.
Feeding
Feeding corals varies drastically depending on the type of coral. Some never eat anything and simply use light and nutrients from the water. Others are heavy feeders and will even take surprisingly large pieces of meaty foods. Most corals will actively feed on some sort of meaty food. Food size is directly proportional to mouth size. SPS eat small foods such as very small plankton, oyster eggs, etc. LPS can take larger foods including brine shrimp, mysis shrimp, small chunks of meat such as shrimp and fish, and even pellets.
Most reef keepers don’t actually feed their corals, the corals get what they need from what is already fed to the fish. Most reef tanks, at least once stocked well with fish, contain a variety of fish that eat a range of food sizes. This is ideal for corals. Many frozen foods for reef tanks contain a wide mix of foods. These aren’t good when you are first starting off, but once you build up the right diversity, these foods can be perfect. They provide a wide mix of foods for every possible inhabitant of a reef tank. So as long as you have something that will eat all of the foods in the mix, they are the ideal frozen food for a reef.
For more information on feeding corals, please read the Feeding Corals article.
Aggression
Coral aggression may seem like a very bizarre term if you haven’t worked with corals before, but it is a very real issue in every reef tank. Physical space on a reef is an absolute premium, like real estate in Manhattan. Corals literally fight to the death over centimeters. Some are much more aggressive and dangerous than others. Some have to come in direct contact, some have far-reaching tentacles that they use to clear space before the main body of the coral even reaches it. LPS tend to be the most aggressive overall, partly because their large polyps allow them to have the farthest reaching stinging tentacles. Euphyllia spp. such as frogspawn, torch, hammer, and anchor corals are some of the worst. They have strong stings that are more powerful than most other corals can handle, and they have very far-reaching stinging tentacles. Give them a wide margin so that even as they grow they can’t reach their neighbors.
Other corals rely on their toxicity if consumed. Zoanthids and palythoa contain palytoxin, one of the most toxic poisons ever discovered. A quick Google search can produce a long list of incidents where aquarists and even their entire families have been hospitalized. I have even heard one case where a curious dog ended up head first in a tank and later had to be treated. It is usually a problem when the corals are being fragged. It can even become aerosolized and affect everyone in the house. The good news is that even though these are so potentially toxic, it is surprising how few cases of palytoxin poisoning occur considering how many aquarists have them in their tanks (effectively every reef aquarist). Palytoxin isn’t just a problem to humans, a recently fragged colony has the potential to wipe out an entire tank of corals and fish.
What’s a Frag?
If you get into corals you will quickly start hearing the term frag. Because most corals grow in colonies that can be broken into pieces and all of those pieces can live, people will intentionally cut off pieces and sell them. These pieces are called frags (short for fragments). Stores will buy colonies that may sell for $100, but instead they cut one colony into ten pieces that can all sell for $15-20 each. More people get a piece, the store makes more money, and everyone who got a piece paid a lot less than they would if they had to buy the whole colony. Having worked in a store, I can tell you that the vast majority of aquarists will not buy a whole colony. The frag will grow into a full colony surprisingly quickly, so the aquarist still ends up with a nice full colony, but instead of paying a lot more, they paid less and get the joy of watching the coral grow on its own. Obviously, the cost of frags varies based on type and size. A large frag of green zoas may only cost $10, while a certain zoa can cost $200 per polyp, and a certain SPS may cost $500 for a piece smaller than the size of a dime. Like anything else, corals are worth whatever someone is willing to pay for them.
Chemistry
Chemistry is important for any reef tank. Nutrient control is vital to keep algae under control (nitrate and phosphate). All corals and invertebrates require a very narrow range of parameters to thrive (pH, temperature, salinity, etc.). Stony corals also require correct levels of calcium, alkalinity, and magnesium for proper growth.
Saltwater chemistry is too important to rush through in just a few paragraphs here. For more information about saltwater chemistry, please read the Saltwater Chemistry article.
Soft Corals
Soft corals tend to be the easiest since they are the least demanding of light, are easy to feed, and they like ‘dirty’ water (this term needs to be used very carefully because it is not actually dirty, just a little dirty relative to the very clean conditions that can be found in some reef tanks). Some of the most common soft corals are mushrooms, zoanthids, Xenia, leathers, and green star polyps.
Mushrooms come in four types, best described by their genus names: Rhodactis (fuzzy mushrooms), Discosoma (smooth mushrooms), Ricordea, and Amplexidiscus (elephant ear mushroom). The Amplexidiscus can be up to a foot in diameter and can potentially eat fish, so in general I would avoid them. All of the others are brightly colored, come in a very wide range of colors, are hardy, perfect for beginners, and are a great addition to almost any reef tank. If they lay across other corals they can burn those corals. This is usually the case when mushroom and zoanthids are placed close to each other and as the mushrooms grow larger and/or move they come in contact with the zoanthids. This can also happen when mushrooms encroach on other corals as well, such as Montipora. Other than this, they are trouble-free. You do not need to feed mushrooms, but some Rhodactis may eat small meaty foods or pellets if they land directly on them.
Zoanthids are colonial polyps that spread across the rock, are roughly a half inch in diameter, and usually stay connected at their bases. They grow more slowly than mushrooms. They do not eat anything and get all of their energy from nutrients in the water and light (although some aquarists report that their zoanthids are happy to eat anything that lands on them). Although they can thrive in relatively low light, they are most colorful if kept under strong lighting. They can even receive the same strength lighting as the most demanding SPS if they are allowed to adjust to it slowly. This will produce the most spectacular zoanthids you will ever see. Zoanthids can be victims of zoanthid nudibranchs and zoanthid eating spiders. Both are important to keep an eye out for and remove immediately if you ever see one or you risk losing all your zoanthids. Most nudibranchs look like whatever they eat. Zoanthid nudibranchs look like a closed zoanthid with just a few tentacles sticking out. The spiders are more reclusive and harder to spot between the polyps.
Palythoas are very similar to zoanthids, but they are significantly larger, most up to an inch in diameter. They have smaller, shorter tentacles relative to their disk size when compared to zoanthids. Their care is exactly the same as zoanthids. They can even be found on the same rocks as zoanthids. However, this frequently results in the palythoas taking over since they tend to grow taller than the zoanthids and effectively shade them out.
Xenia is a very popular soft coral and great for beginners. They grow so well that many people actually consider them pests. They can move further and more quickly than almost any other coral. They spread rapidly. Although their coloration barely varies (somewhat from species to species, but mostly based on the lighting), what makes them so popular is that they actively pump or pulse, they actually contract and relax their tentacles. This creates a hypnotizing and relaxing rhythm. White pompom Xenia is the best at actually pumping, most other varieties stop or greatly reduce how much they pump, especially if kept in any decent flow. Xenia is considered a pest by many people because it usually does very well and will start spreading. If it does start spreading to areas you don’t want it, just grab it by the base with hemostats and peel it away from the rock. You can toss out the unwanted Xenia, or get it to attach to some rock and sell it to other reef aquarists or a local fish store. To avoid a takeover, keep the Xenia on a single rock or isolated rock pile and immediately remove any that show up anywhere else in the tank (Xenia can detach and drift to other areas in the tank). I like to put them against the back glass and let them grow all over the back wall. They will take over and smother almost any coral.
Leather corals are very large soft corals and come in many shapes and a modest selection of colors (usually some variation of brown, yellow, or green). Their size and shapes make them attractive options for many reef tanks. They don’t eat much of anything beyond light and nutrients in the water. Be aware though, they do molt their skin and can give off a mild toxin that SPS don’t like. This doesn’t necessarily mean they can’t go together (some of the best examples of each I have ever seen were in the same tank), but it may mean that you need to use carbon to keep the toxins in check and do an extra water change after one molts.
LPS
LPS are a favorite among many reef aquarists because of their size and the fact that so many of them flow and sway with the current. The most popular are Euphyllia (torch, frogspawn, hammer, and anchor), Acans, Brains (which actually includes a lot of different genera of corals), Favia, chalice, and many more. They are colorful, large, move with water flow, are less demanding and more forgiving than SPS corals, and most are relatively hardy.
LPS do require calcium, alkalinity, and magnesium to build their skeleton. Once you have any LPS, you should start testing these to ensure they are in the ideal ranges. You may not have to start dosing right away, but testing will let you know for sure that you don’t need to dose yet and tell you when you need to start.
The usual problems with LPS are lighting, flow, and space. Some aquarists get LPS before they should. Their soft corals do well enough and they take the next step. They soon learn that although their lighting was enough for soft corals to get by with, the LPS go downhill or at least have very reduced colors and little to no growth. Flow is important in a reef, it provides food and oxygen while removing waste, but too much flow can be a problem for LPS. Due to their large polyps, LPS can become very irritated with strong flow. Think of a moon bounce on a windy day, there is so much tissue that the flow can tear apart the polyps. The only defense the corals have, and the way you are most likely to realize you have the problem, is the polyps will stay closed or will only partially open. As for space, LPS can be very large, grow quickly, and most problematically, they can have far reaching tentacles. Placing them too close to each other or other corals can have disastrous results.
LPS are generally eager feeders and will open up their feeding tentacles whenever you add food to the tank. They can take surprisingly large pieces of food, limited only by their mouth size. Depending on the type, they will take brine shrimp, mysis, krill, large chunks of fish and shrimp, and even pellets. I usually feed my LPS directly on a weekly basis. I feed either pieces of shrimp or actual pellets. This keeps them happy and growing well.
SPS
SPS are among the most demanding of all corals. They require the strongest light, great flow, clean water, and a great supply of calcium, alkalinity, and magnesium. They can be very fast growers.
The easiest SPS are Montipora. These are definitely the best first SPS. They come in three main types, encrusting, plating, and digitata. Encrusting montipora simply grow on top of the rock they grow on. Plating Montipora will grow out away from the rock in large, wide cups which can grow to be quite large. Digitata look like encrusting, but they begin to grow irregular projections out into the water column. The projections are shaped similar to branching SPS, but the growth pattern is more irregular and asymmetrical. Fortunately, Montipora come in a wide range of colors, some of which rival even the best Acropora spp. Their colors include green, red, purple, orange, blue, and many combinations all the way up to rainbow which actually includes all colors.
Birdsnest is another great entry-level SPS. Unlike montipora, birdsnest only grows out in a fine, interconnected branching that has the familiar branching growth pattern most of us think of when we think of corals. Although birdsnest does come in a wide variety of colors (including green, purple, pink, yellow with purple polyps, and more), they aren’t quite as varied or as bright as montipora can be. Another great thing about birdsnest is that if you buy a large enough colony (not just a tiny frag) you stand a decent chance of also getting a coral crab along with it. Birdsnest (as well as other branching SPS such as Acropora spp.) can house commensal crabs. The crabs don’t hurt the coral, so don’t worry, they are just another interesting addition to a reef tank.
Beyond the Montipora and birdsnest, you get into the most demanding of all corals: Acropora spp., Stylophora spp, Pocillopora spp., etc. These are the ultimate corals. They are the most demanding of light, flow, calcium, alkalinity, magnesium, and water quality. I won’t go into too much detail here, advanced care for these corals can literally fill a whole book. If you are at the point with your reef where you are considering these corals you will definitely want to do more research before taking the final step of buying any. If you have stepped up from the easier corals to the more demanding corals and they are all doing well, including other SPS, then you are likely to do well with these corals.
Clams
Clams are not related to corals at all, but their care is identical. They require very strong light and an endless supply of calcium and alkalinity, which is why you may hear them referred to as calcium sponges. Once they hit a certain size (two inches or so) they effectively don’t eat at all. Under this size, they are so demanding of food that they starve in all but the most devoted clam keepers’ care. They can also have a surprisingly positive impact on water quality. Because they don’t eat anything once they are larger than about two inches, they rely on nutrients in the water (nitrate and phosphate). They can have such an impact that you can compare them to a modest refugium when it comes to nutrient export. They attach to the rocks through threads that come out of the bottom of their shell (called byssal threads). Clams have eyespots along the border of their mantle (the soft tissue). This is most obvious when they close when something casts a shadow over them. They are easy victims to nippy fish such as angels (the usual culprits are dwarf angels such as flames and coral beauties which really shouldn’t be in a reef tank in the first place). The mantle houses zooxanthellae and is therefore opened up during the day so that the clam can ‘feed’ on the light.
There are only a handful of species commonly available in the hobby, and they are all within the Tridacna genus. They are derasa, squamosa, maxima, and crocea (plus any new species, divisions, or reclassifications that have been published since writing this paragraph, something very common in this hobby). In general, they are best identified by their shells, not their mantle. However, there are general trends for the pattern and coloration within each species (although when in doubt, and to be sure, go by the shell).
Derasa are the hardiest, most forgiving, and the best option for a first clam. They tend to have white marks which vary from thin marks closely arranged to wide marks separated by much more brown. These get the largest of the common four species maxing out at around 20” and are characterized by a perfectly smooth shell. These are generally found on the bottom of slightly deeper water than the other clams, which is why they are more forgiving of less than ideal lighting. They are great on the sand (if your lighting is strong enough), but are happy to attach to rocks as well (usually a good idea in most tanks, depending on light).
Squamosa clams may look like a derasa at a quick glance, but have more variability in their color, their markings are much rounder, and their shells have the most pronounced scutes (the projections on the outside of their shell). They are also very large clams, maxing out at only a few inches shy of a derasa. They do have more pattern and color variation though, including blues that will lead you to think they are crocea.
Maxima and crocea clams are very similar. Crocea tend to be blue (although not strictly) whereas maximas tend to be gold (although not strictly). Both are found in the most shallow water in the wild, embed themselves in the rocks (if they grow up inside the rock, starting in a crack or crevice), and are the most demanding of lighting. Maxima tend to have fairly pronounced scutes (although not nearly as pronounced as Squamosa), whereas the crocea tend to be almost smooth and vary from mild scutes to being as smooth as derasas.
Although they are demanding, clams are not as bad as you might expect. They aren’t as strict about water quality as some of the more demanding SPS. The biggest limiting factor for them is light. If you don’t provide strong light then they won’t do well. This doesn’t mean you need metal halides or the most expensive LEDs, you just need a lot of good light. My first reef tank had compact fluorescents and both of my clams were very happy, colorful, and grew quickly. My current reef has T5HOs and my clams are very happy and growing well. Whatever type of light you use, you have to have a lot of it. If you are doing well with basic SPS such as montipora or birdsnest, then you will probably do well with clams. Be sure to watch your calcium and alkalinity as well, they will suck it out so fast you won’t believe it.
If you are interested in further information about clams, you can’t do any better than James Fatherree’s book Giant Clams in the Sea and the Aquarium. It goes into great detail about their anatomy, physiology, nutritional requirements, lighting, etc.
Anemones
Anemones are another non-coral group that have care requirements very similar to corals. They are more particular about their care though, so just because you have corals doing very well doesn’t mean you will succeed with anemones nearly as easily. Anemones are also mobile and will move around until they find the right spot with just the right balance of light and flow. Until they stop moving, everything in their path is subject to their damaging and often lethal sting.
The general rule for anemones is that they should not go into a reef tank that hasn’t been running for at least six months. And if you have had any recent issues such as trouble keeping chemistry stable, algae blooms, etc., then you should probably wait even longer.
Anemones do actively eat and should be fed large meaty foods on a regular basis, ideally weekly. The usual food is something such as large shrimp or silversides. This will keep them happy and growing well. The exact size of the food to feed will vary based on the size of the anemone. Obviously a 2” bubble tip anemone can’t handle the same size food as a 12” carpet anemone.
Bubble tip anemones are the best option for a first anemone. They are small, more affordable, hardier, and more likely to survive than other types. They can easily be found in green or red (which can vary from dull orange, to red, to neon pink). As with corals, their color will vary based on lighting.
You will need to do more research on their care before purchasing any anemone. They are not for beginners. Many need much deeper sand beds than most reef tanks have. Few are proper hosts for most clownfish, and even then many clownfish won’t take to them right away, if at all (especially if captive bred). So if you only want a reef to have a clownfish and anemone together you may be disappointed.
Growth
Growth rates can vary drastically between different types of corals and in different conditions. Sometimes it makes perfect sense, such as when you start testing and dosing calcium, alkalinity, and magnesium and all of a sudden all your stony corals grow a lot faster. Sometimes it doesn’t make any sense at all, such as when all of your Montipora or zoanthids grow well except one particular type. Don’t worry about it too much. Make sure you are doing things properly (lighting, dosing, etc.), beyond that, it is up to them.
Lighting is most important to growth. No matter how good everything else is, if you don’t have good lighting, you won’t have a thriving reef. For stony corals, the next most important factors are calcium, alkalinity, and magnesium. Stony corals simply can’t grow well without adequate calcium, alkalinity, and magnesium. The final major factor is feeding. Feeding is more important to certain corals, you will usually see the most improvement in LPS. You are likely to see much better growth in LPS that are directly or spot fed meaty foods or even pellets. Although other corals will actively eat, other factors play a bigger role in their growth rates (nutrients in the water for zoanthids and mushrooms; lighting, calcium, alkalinity, and magnesium for SPS; etc.).
It is very common for corals to not grow much at all when they are brand new to the tank. All types of corals may sit idle for a while and then all of a sudden start growing. You may see nothing for weeks, then notice a little new growth, and a week later there is significant growth. Once they settle in and get in gear, they can really start growing quickly.
Buying Corals
It is always best to support your local fish stores. As someone with NO STORES WITHIN 1.5 HOURS, I can tell you how important is to give them your business. Nothing compares to seeing frags in person. Although I am willing to buy online for the right things, it is not without risk. My first online purchase was four different Rhodactis mushrooms, simple enough, right? Well, apparently not. Two of them weren’t even Rhodactis, they were discosoma (although, in their defense, one was so small you could barely tell it was a mushroom).
A second option, and still keeping things local, is to look into your local aquarium clubs and Craigslist. Many clubs have frag swaps, and people are constantly selling and swapping frags on their forums. Craigslist can be hit or miss, but there is a good chance you will find something good.
If you decide to buy online, try these tried and true suppliers:
Summary
Corals can be as challenging as you want them to be. You can have soft corals that take over like weeds, or you can strive for maximum growth and coloration from the most demanding SPS. Either way, a reef tank full of corals is almost guaranteed to make you happy and bring you an endless supply of enjoyment.