The phylum Cnidaria contains more than 9,000 species that all have one thing in common: They all have cnidocytes, which are stinging cells used to capture their prey. There are many members of this diverse phylum that are important to aquarists, including stony corals, soft corals, sea anemones, corallimorpharians and zoanthids. More and more people are setting up smaller reef aquariums, and cnidarians have a special place in nano-reef communities.
Selecting cnidarians for a nano-reef can be a tricky proposition. For example, because room is at such a premium in the nano-reef, you will have to pay even more attention to the growing characteristics of a coral to determine if it will work in the mini-reef. You must also carefully consider the environmental requirements of the species you hope to keep. For example, some species require intense lighting to thrive. While there are now a wide range of lighting systems available for small tanks, you may not be able to put enough illumination over a nano to keep certain corals and clams. The same applies to water movement. While it is possible to retrofit extra water pumps in a nano-reef, many of the nano-systems you commonly see at stores lack adequate flow for certain types of corals.
These are just a few things to consider when selecting cnidarians for a nano-reef.
I want look at some select cnidarian species that are good choices for the nano-reef aquarium. We will start our survey with the stony corals.
Reef aquarists have access to more than 100 different stony coral species. Many of these can form very large colonies if they have enough space. This means that most corals will eventually outgrow our tiny glass boxes. Therefore, when selecting corals for our nano-reefs, we need to concentrate on species that are easily pruned. It is like the art of Bonsai — we cut back our corals as they grow to keep them at a manageable size. If you are willing to engage in frequent pruning, there are many corals that will work in the nano-reef. One thing you need to do is to start with smaller specimens (i.e., frags). (For more information about fragging corals, see the “Want More Info?” box at the end of this article for details.)
Stony corals extract trace elements out of the water, which can quickly be depleted in the limited volume of a nano-reef. To keep trace element levels at natural levels, frequent water changes (20 percent per week) are needed. This will also help remove pollutants that build up in these closed systems.
I suggest that nano-newbies steer clear of small-polyped stony corals (SPS). Many of these animals are more dependent on high calcium and alkalinity to thrive. (Calcium levels should be between 380 to 450 parts per million, while alkalinity should be around 2.8 millequivilents per liter.) It can be difficult to maintain calcium levels in a small aquarium unless you are willing to check the water chemistry frequently (several times a week) and add calcium and a buffer (supplement added to water to maintain high alkalinity) as needed. While some hobbyists may be willing to make this commitment, many will not invest the necessary time and energy, and will end up with unhealthy or even dead SPS corals. If you want to give the SPS corals a shot in your nano-reef, remember that the more specimens you have, the more calcium they are going to utilize.
Large-polyped stony (LPS) corals are much better-suited for the nano-reef. While they also rely on calcium, their skeletons don’t grow as rapidly, so it is easier to maintain adequate levels (the water changes should keep the calcium level high enough for these corals). The only drawback with some of the LPS corals is that they can be rather aggressive. In most cases, they express their pugnacity by stinging neighboring corals with stinging structures (e.g., mesenterial filaments, which are structures used by corals for defense, feeding and cleaning surrounding substrate to encourage growth).
Large-Polyped Stony Corals
“Acans.” The Acanthastrea (usually referred to as “acans”) are some of the most spectacular and expensive of the LPS corals. Acanthastrea lordhowensis is one of the most popular species, sporting a variety of amazing colors and patterns (the more colorful, the higher the price). Not only are many of the acans attractive, they are also not overly aggressive. They do have mesenterial filaments, but they are not nearly as long as the sweeper tentacles of some of the more “territorial” corals (e.g., Galaxea).
Acanthastrea corals can withstand a variety of aquarium conditions. They do best under moderate to high lighting (power compacts, T5 fluorescent or metal halide lights) and moderate current. If you keep them under less-intense lighting, place them closer to the light source.
To increase growth rates, you can feed these corals meaty foods (e.g., minced seafood, Mysis shrimp). These aquarium foods are best deposited on the coral polyps at night several times a week. It is also best to feed them when current sources have been turned off for a short time (do not forget to turn them back on). These corals will also consume food particles added for their piscine tankmates. They do not grow fast; and if they grow too large for your tank, they are easy to frag.
Blastomussa. Known commonly as pineapple corals, members of the genus Blastomussa are also popular for nano-reefs. Most are shades of green, brown or reddish-brown. When the fleshy polyps are extended, you cannot see the stony skeleton. They tend to occur in more sheltered areas of the reef and thus are not appreciative of high-current microhabitats in the aquarium. They also do fine under moderate illumination.
Pineapple corals are slow-growing and tend to do best (especially when first added to a tank) if they are fed meaty foods. They are also not aggressive, and they have a tendency to be “bullied” or overgrown by more pugnacious corals.
Caulastrea. Members of the genus Caulastrea are durable additions to the nano-reef. The most popular species is C. furcata, which is most often sold as the trumpet or candy cane coral. These corals do best with moderate water movement and moderate to high lighting (T5s, power compacts or metal halides are fine). They have short sweeper tentacles and are not considered aggressive. Caulastrea furcata is also easy to frag (simply break or cut the larger corallites apart).
Euphyllia. Euphyllia are commonly housed in larger aquariums, and many species can form large colonies. There are a number of species in the genus; my favorite for the nano-reef are the hammer or anchor corals (E. ancora and E. paraancora). These beautiful corals have long tentacles that extend from a large, fleshy polyp that can be gray, brown or bright green. In the case of E. ancora, the corallite is elongate (which are difficult to frag), while E. paraancora has trumpetlike corallites that can be easily clipped or broken off of the “mother” colony.
The Euphyllia corals benefit from a lively, indirect current (they should not be pummeled by a direct jet of water). They will ingest smaller food particles and do best under moderate to high light conditions.
The only downside with these corals is that they tend to have long sweeper tentacles that will damage neighboring cnidarians. Therefore, you may want to make this the “focal species” (that is, keep it on its own) in a smaller nano-reef (e.g., 5 gallons). When searching for a specimen for your tank, find a frag from an established captive colony. Never purchase a specimen in which the tissue is pulling away from the skeleton, or where dead tissue or brown jellylike material is present on the polyps.
While many of the more durable forms are not that colorful, soft corals add form and motion to the nano-reef. Most prefer strong water movement to help rid them of the slime they constantly produce. They shed this mucus to rid themselves of fouling organisms and debris. If water does not flow over them, accumulating slime can interfere with feeding and respiration.
Soft corals tend to grow faster than their stony brethren — this means frequent pruning is essential to keep them in check in the nano-reef. They also commonly reproduce asexually, dropping branches and tissue from the “mother” colony. One possible downside to keeping soft corals in a closed-system is that they produce metabolites that may be harmful to other corals and even to fish tankmates. These chemicals serve to prevent other corals from growing too close to them. If you can use a high-quality carbon in your system, a protein skimmer and do frequent water changes, you will be able to keep these substances in check.
So what species of soft corals should you consider for your nano-reef? There are a variety of soft corals lumped together under the common name “tree corals.” Most of these belong to the genera Sinularia, Lemnalia and Litophyton. These tend to be larger corals that will grow too large for the nano-reef, but many species are easily pruned. So, if you are willing to do some cutting, they can be kept at a small enough size to survive in a smaller tank. Good lighting (at least power compacts or T5s) and good water movement are essential to maintain good health.
Sarcophyton. The toadstool-like leather corals are also wonderful nano-tank residents when they are small, but most grow too large. When they begin to fill up a nano-reef, simply cut the leather coral off near the base, and it will grow back. Take the large section you removed from the tank, and sell it to a friend or trade it at a local aquarium store. These corals tend to be quite hardy if given an appropriate amount of illumination (they should be housed under moderate to bright lighting). They also need moderate amounts of water flow. While most Sarcophyton species are tan or cream-colored, there are some highly sought-after yellow and green species.
Xenia. The members of the genus Xenia are popular because of their waving, flowerlike polyps that are hoisted up on sturdy stalks. They are somewhat “chromatically challenged,” typically exhibiting tan or cream hues, but the polyps of at least some species pulse (rhythmically open and close).
Keeping Xenia can be a rather dubious endeavor. In some systems, they grow out of control, often smothering sections of bare live rock and blanketing the aquarium sides. In other systems, they turn to mush. The conditions in your aquarium and the quality of the specimen you acquire will determine how the coral does in your nano-reef (always purchase captive-raised individuals).
These corals need strong water motion, good lighting (moderate lighting like power compacts and placement near the top of the tank) and iodine supplementation. They also tend to do better in aquariums with higher dissolved nutrients (a condition often present in nano-reefs).
Clavularia and Pachyclavularia. Members of the genera Clavularia and Pachyclavularia (known commonly as clove polyps and star polyps) are also popular nano-cnidarians. They can be gray, brown or bright green (on rare occasions, they may have a bluish tint). These animals grow well under moderate lighting and with moderate water flow (they do not do well if beaten by a direct jet of water). They grow rapidly, and while they are not potent stingers, they may overgrow more benign corals. It is important to clean detritus off of the matlike stolons (slender horizontal branch serving to propagate the organism); otherwise, these structures become a site of profuse hair algae growth. Frequent water changes will be required to replenish the trace elements that these corals need to thrive.
The corallimorphs (aka “mushroom anemones”) are some of the most popular cnidarians for the nano-reef. They come in a variety of different hues, color patterns, textures and sizes. Most species encountered in the aquarium trade are often found in nutrient-rich, protected coastal habitats, and thus are well-suited to captive conditions. Another corallimorph benefit is that they don’t tend to be overly aggressive. This is especially true of the smooth varieties, such as Discosoma species. Those species that are more frilly (namely members of the genus Rhodactis, which are usually sold as “hairy mushrooms”) can be potent stingers, while these and others have also been known to exude mesenterial tentacles from their mouths to fight off and even consume cnidarian neighbors.
The durability of members of this group varies, but most are hardy. The most-demanding species belong to the genus Ricordea (e.g., R. florida, an Atlantic native). These more sensitive forms are most often collected in the reef shallows (often in turbid conditions) and tend to prefer more intense illumination. Many Discosoma and Rhodactis species do better in low to moderate light venues. When light levels are too low (e.g., when bulbs need to be changed), they will extend toward the light — a phenomenon known as trumpeting.
The corallimorpharians don’t like strong flow rates and are more likely to thrive in the “turnkey” nano-reefs systems available at aquarium stores (I have found that many of these systems do not have adequate water movement for some of the current-loving cnidarians). They also will utilize some of the organic nutrients that are often abundant in closed systems, often growing and reproducing at a slower rate in systems where nutrients have been stripped by skimming and chemical filtration. These animals regularly reproduce asexually, and may take over sections of an aquarium or the whole tank if conditions are optimal.
The zoanthids are also a staple for nano-reefkeepers and for good reason. They are hardy animals that come in an amazing assortment of colors, including green, blue, red, orange and a mixture of these colors. In fact, hobbyists have come up with some creative monikers to describe the various colors, such as “nuclear green,” “cherry bomb,” “sparkling pink explosion” and “flaming fire.” They occur in a variety of reef habitats and rely on light (like the other cnidarians covered here, these animals rely on zooxanthellae that live in their tissues to provide some required nutrients); they also feed on bacteria, dissolved organic materials and capture very small plankton.
These animals can cause problems for their neighbors, as they produce toxins and can overgrow more passive coral species, so make sure you give them enough space. Most do well in the confines of a nano-reef with moderate (minimum of power compacts or T5s) to strong lighting (under moderate lighting, place them closer to the lights) and moderate to strong current.
That concludes our brief look at some of the best cnidarians for the nano-reef. Nano-reefs can provide a fantastic window into the lives of saltwater fish and invertebrates for a greater number of people because they are less expensive to obtain, set up and maintain. Hopefully, this increased exposure will inspire more of us to get involved in reef conservation issues. If you want to glean more information on these animals, check out the books listed in the suggested reading section.
Broneman, E. H. 2001. Aquarium Corals: Selection, Husbandry and Natural History. TFH, Neptune City, New Jersey. 464 pp.
Calfo, A. 2007. Book of Coral Propagation: Reef Gardening for Aquarists, Vol. 1, Ed. 2. Reading Trees, Monroeville, Pennsylvania. 416 pp.
Sprung, J. and J.C. Delbeek. 2005. The Reef Aquarium, Vol. 3: Science, Art and Technology. Ricordea Press, Coconut Grove, Florida. 680 pp.