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HOW TO CARE FOR YOUR ACROPORA CORAL
Acropora are the crown jewel of the SPS world. No other genus of coral has the sheer number of species as Acropora, and when reef aquarists talk about the care requirements of an SPS system, they are really talking specifically about the husbandry of Acropora. This is understandable given the challenging nature of Acropora. They are one of the most difficult corals to keep, and sets a high standard for husbandry requirements.
Let’s look at what makes Acropora more difficult to care for. To put it simply, they are highly sensitive to changes in water chemistry and are demanding in terms of flow and light. Unlike more forgiving corals, Acropora demonstrate their displeasure in one of two ways. They can either change color to something unappealing in mild cases, or they outright die right before your eyes in the severe cases.
Many experienced hobbyists struggled keeping Acropora long term, but that ironically is part of the attraction to this coral. Taking on new challenges and improving ones husbandry is a good thing in this hobby and successfully keeping Acropora serves as a milestone of success. The challenge of keeping Acropora is tied to their high demands for lighting, water flow, and pristine water chemistry. The challenge is compounded by their need for incredible stability of those parameters. On the surface it sounds simple but those that have successfully kept Acropora understand the underlying difficulty.
All these things, lighting, flow, and proper chemical levels are a moving target. A fast growing SPS reef is dynamic in nature, and today’s success directly leads to tomorrow’s failure. The lighting a coral receives increases for parts of the colony that extend upwards towards the light while simultaneously shading parts of its own structure and all the corals unfortunate enough to be below it. An Acropora colony’s growth can stifle flow to other nearby corals or even to itself as it gets larger. Similarly, the uptake of major and minor elements as the corals successfully grow can outpace the hobbyists day to day methodologies and suddenly full tank crashes can occur.
Given the challenges outlined above, why are they so popular in the reef keeping community? For many reef aquarists, an SPS dominated tank full of Acropora is love at first sight. Although the SPS tank often lacks the mesmerizing movement of other types of reefs filled with Euphyllia or anemones, the Acropora dominated tank is a bright explosion of color where each nook and cranny is its own firework show.
Acropora come in every color imaginable and their color palate is highly variable. A colony grown in one system can change color dramatically when moved to another system. I occasionally visit friends that purchased an Acropora from me and I am amazed at how much it has changed in their aquariums. It is completely unrecognizable from its time in my coral farm. This is especially true if they are doing something completely different in their system such as running an ultra low nutrient system for example.
The color variability of Acropora also feeds the desire to chase “optimum coloration.” With these corals it is not good enough to have them survive and grow, but to also express the most aesthetically pleasing colors. The lengths that experienced SPS keepers will go to to have ultimate coloration is staggering.
Location
Indo-Pacific - Acropora are a genus of small polyp stony corals found in reefs throughout the world including the islands of the Indo-Pacific including Fiji, Tonga, Solomon Islands, and the Great Barrier Reef. They grow branching colonies that take on a variety of forms ranging from stag horns, elk horns, or even flat tables. Acropora are one of the primary reef building corals and are responsible for a large percentage of a reef’s structure.
Lighting
Most coral on the reef are photosynthetic and have some demand for light. Like many corals, Acropora have a special symbiotic relationship with dinoflagellates called zooxanthellae that live inside its tissue. The dinoflagellates are actually the photosynthetic organism and the coral animal consumes the simple sugars that are byproducts of photosynthesis. Zooxanthellae is usually brown in color and the coral tightly regulates the population living in its flesh. Too little light will cause the coral to turn brown in color. As it seeks more nutrition, the coral allows more zooxanthellae to build up in its body. Too much light and Acropora will expel the zooxanthellae making its overall appearance lighter in color. If a coral is particularly stressed it can expel nearly all its zooxanthellae as a last resort and cause unhealthy bleaching. Hobbyists looking to find that “just right” color play with both lighting intensity and spectrum over their tank.
There is a misconception in reef keeping that all corals require high lighting. In fact, very few corals need high intensity lighting and in many cases problems arise when there is too much light not too little. Acropora however are one of the few types of coral that are truly light loving. In our systems Acropora have fared best when given light intensity around 300 PAR however there are plenty of successful systems with lighting intensities higher than 500 PAR. Having said that, I don’t recommend blasting newly added Acropora with a ton of light right away. More damage is caused by overexposure to light intensity than not providing enough light so take a couple of weeks to allow the coral to adjust to lighting conditions in your tank.
Low Light
Low light translates to about 30-50 PAR
Medium Light
Medium Light is between 50-150 PAR
High Light
High Light is anything over 150 PAR
Lighting is a loaded topic, so for a more in-depth discussion of lighting, please see our Deep Dive article and check out our video below:
Water Chemistry
There are three major chemical parameters that are needed by Acropora to build its stony skeleton. These are Calcium, Alkalinity, and Magnesium. Calcium is one of the major ions in saltwater. In most healthy reefs, the calcium level hovers around 425 parts per million (ppm).
Alkalinity is a little more difficult to explain than calcium. It is not a particular ion, but can be thought of as the buffering capacity of saltwater. Buffering capacity is the amount of acid required to lower the pH of saltwater to the point bicarbonate turns into carbonic acid. It sounds over technical, but in layman’s terms, higher alkalinity levels equate to greater chemical stability in our reef tanks. In practice alkalinity tends to be the parameter that fluctuates the most of the three and is the one that needs the most babysitting. In the wild, the alkalinity of the water is around 8-9 dkh.
One quick note about adjusting calcium and alkalinity: it can be a little tricky because of how they interact. For example, if your reef tank had a calcium level of 300ppm when you desire a value closer to 400ppm, you could theoretically add a calcium supplement to boost it. Unfortunately, reef aquarium chemistry is dynamic and solutions to chemistry issues are rarely that straightforward in practice. Addition of a calcium supplement in this manner often comes with a corresponding fall in alkalinity levels.
This see saw effect between calcium and alkalinity stems from how the two ions interact with one another. The two ions combine to form calcium carbonate and fall out of solution, thus lowering both levels.
If you are experiencing this in your systems, the possible culprit with calcium and alkalinity instability is Magnesium. It may seem counterintuitive that the solution to calcium and alkalinity imbalances is to elevate magnesium, but the three ions interact regularly.
Magnesium is very similar chemically to calcium. It can bind up carbonate ions thus increasing the overall bioavailability of alkalinity compounds in the water. So again if you find that no amount of tweaking calcium and alkalinity directly is helping, you may want to make sure it is not your magnesium level that is in fact low.
Having said all that about tweaking levels of calcium, alkalinity, and magnesium to something resembling natural salt water levels, it is not something you want to do in a knee jerk fashion. Acropora respond very poorly to sweeping changes in chemistry. For example, if alkalinity dips, they make take on a brown coloration and stay that way for months even if the alkalinity level is corrected. The best advice I can give is to strive for consistency even if the values on the test kit are not ideal and slowly… very slowly try to raise them over the course of weeks or months.
Again, just maintaining levels is tricky. Successful Acropora filled tanks experience rapid growth, and larger colonies soak up calcium, alkalinity, magnesium and trace elements at a much faster rate. This is why for my systems here I like to provide as stable a baseline as possible through water changes and calcium reactors and then add chemical additives when necessary.
Water Cleanliness
As far as water cleanliness goes, two parameters to keep low are nitrate and phosphate. Elevated phosphates can lead to poor coloration and possible algae issues. Nitrate is an indicator of poor water quality and can cause stony corals to crash altogether if not lowered. The natural sea water levels of nitrate are between 5 ppm and 40 ppm. For Acropora, it is best to be on the lower end of that range. Phosphate levels should be much lower (around .01 ppm) but I would caution hobbyists that are looking to keep those two parameters as close to zero as possible. Nitrate and Phosphate are not bad in and of themselves. Elevated levels of them can cause problems, but they are absolutely required for biological processes in coral and cannot be produced through photosynthesis.
Water Flow
Acropora are found in some of the strongest current areas of the reef and benefit greatly from strong water movement in the home aquarium. Water movement is essential for bringing nutrients to coral and more importantly removing waste away from them. Acropora even grow in patterns to adapt to the flow in a given area. For example, Acropora in very strong flow grow thicker and more dense than in tanks with less flow. Some species of Acropora might even take on a stockier shape with fewer long branches in very high flow areas.
The growth of the colony in relation to flow also plays a part in their nutrition. They might be growing in such a way to maximize bacterial growth between the branches. One publication that I found interesting was from Coral Reef in 1989 by Schiller and Herndl. Basically it took a look at the interstitial space around certain SPS. They looked at a few different parameters such as ammonia, nitrite, nitrate, phosphorous, and bacteria levels between the branches on the interior of the colony compared to the ambient water column. What they found was that there were lower concentrations of dissolved organics in the interstitial space with an associated uptick in the concentration of bacteria.
The corals may be feeding on bacteria directly or indirectly attracting microbe-feeding zooplankton that they then trap and consume, but it is interesting that the corals studied grow in a fashion that optimizes flow through the branches to maximize bacteria farming opportunities.
When trying to provide adequate flow there are two things over time that dramatically affect the performance. The first is the growth of the colony itself. Successfully growing Acropora quickly comes with the downside of the coral being a victim of its own success. Large colonies cut down flow significantly and over time choke off flow to other nearby colonies or even to the inner parts of itself. As colonies get larger and larger, it is important as hobbyists to pay close attention to changing flow demands and consider adding more flow or pruning the colony.
Secondly, you may notice that there isn’t quite as much flow as you once had when everything was freshly installed. Other organisms love to grow in and around the aquarium’s pumps and plumbing. For this reason I recommend taking apart pumps and powerheads regularly for servicing. It does not take very much growth or blockages to greatly limit water flow output.
Feeding
We talked earlier of Acropora nutrition when we talked about lighting, but their requirements extend beyond their relationship with zooxanthellae.
Although a high percentage of Acropora’s nutritional requirements are acquired by photosynthesis, they also benefit from regular feeding for both growth and coloration. There are three great sources of food that work well, amino acids, small zooplankton, and simply having fish present.
Starting with amino acids, they are simple organic compounds containing a carboxyl (COOH) group and an amino group (NH2). To quote every high school biology text book, they are the components of proteins that are the building blocks of the cell. In addition to their role building proteins they are also necessary for other biological functions such as neurotransmitter transport and biosynthesis.
The amino acids needed vary on a species by species basis but practically speaking it makes little difference in the long run because even if certain amino acids go un-utilized by a certain Acropora they will be taken up quickly by another organism for their biological process. There are several commercially available amino acid additives so you don’t have to overthink it too much. If you would like to know more about amino acid dosing, you can take a look at the video below:
Small zooplankton include things such as rotifers and cyclops plankton. The rotifers we feed are usually around 0.5mm in size. Cyclops plankton are larger typically between 1-2mm. They come frozen and are basically a small granular oily paste that creates an orange cloud when introduced into the tank. The presence of rotifers in the water is immediately apparent to the corals because many of them will immediately open up and start their feeding behavior. Acropora do this to a lesser degree but what is noticeable are white strings of mucus leading to each of the small polyps. Over the next several minutes, the Acropora corals will retract the strings and feed on the zooplankton trapped.
Last point on nutrition, a higher volume of fish in the aquarium do seem to have a positive effect on Acropora coral colonies. This is a purely anecdotal observation that is also echoed by many other aquarists that have kept SPS tanks successfully. Perhaps their presence as a nitrogen source in close proximity is a good thing.
One caveat I will add about feeding is this. Although coral nutrition is important, it is equally important to make sure to not overfeed the aquarium. Overfeeding can lead to issues such as algae or cyanobacteria blooms that can be a hassle to overcome. A little bit of feeding goes a long way but all of the benefits can be wiped out by a nutrient overload caused by overfeeding so go slow and keep a close eye for signs of nutrient overload such as nuisance algae or greatly elevated nitrates.
Placement/Aggression
Although they don’t look it, Acropora are an aggressive coral. They don’t have stinging sweeper tentacles or engage in chemical warfare but they pack a powerful sting especially to nearby SPS. When two Acropora touch, winners and losers determined quickly.
The video below provides an overview of the different manifestations of coral aggression and ideas on how to mitigate some of the risks inherent in keeping corals in close quarters.
One of the most frustrating things to have happen in an SPS dominated tank is to have one colony get dislodged and fall into another colony below. Even if caught quickly, there can insane amounts of damage in just that short interaction. If the fall was not caught until the next day, it is possible that both colonies could die.
Sometimes it does not even require one coral falling into another. Acropora grow quickly and two colonies can grow in close proximity to one another. I personally have been lulled into a false sense of security until one day the two colonies get a tiny bit too close to one another and I wake up to two colonies that are half exposed skeleton. When thinking of placement, think ahead and give them plenty of room to grow.
Diseases and Pests
Acropora spp. are some of the most common and prolific stony corals found on the reef. They are found in almost every kind of reef habitat and are highly adaptive. They can take on completely different shapes and colors depending on their location on the reef. You would think that they are extremely hardy and can be tossed into any reef setup, but that is not the case. They are sensitive to water quality, and are sometimes stricken with either rapid tissue necrosis (RTN) or slow tissue necrosis (STN).
These ailments plague the tanks of both beginner SPS keepers and experienced SPS keepers alike. What makes these diseases all the more frustrating to deal with is there is no consensus on what causes them. Much of the discussion in the community revolves around fluctuations in chemical parameters especially calcium and alkalinity however there are plenty of examples of aquariums with rock solid chemistry that fall victim to RTN or STN. Other potential culprits include high organic levels (phosphates and nitrates), lack of water flow, and high temperatures. It is a perplexing problem because a colony might be doing well for years in the same tank conditions and one day lose the majority of its tissue overnight. It is very possible that a fluctuation in some tank parameter causes RTN or STN but it is also possible that the Acropora colony tolerated a suboptimal condition for an extended period of time before reaching a sudden tipping point.
As far as treatments go, some hobbyists have had success stopping the tissue recession by dipping the Acropora colonies in an iodine solution such as Lugol’s. Personally I have not had any luck with that approach and resort to quickly propagating the colony in hopes that one of the frags will survive to regrow. What tends to work best is to cut healthy portions of the colony and reglue them to a fresh substrate. Some of these fresh cuttings may not survive, but hopefully some will. It is not an ideal solution, but if left uncut, the necrosis will spread throughout the coral and you will be left with nothing.
Acropora sometimes fall victim to a variety of pests. The two most commonly seen in home aquariums are Acropora eating flatworms and small crustaceans hobbyists call “red bugs.”
Acropora eating flat worms are extremely difficult to see. This is due to their bodies mimicking the color and texture of the coral itself. The only tell tale sign of them is the Acropora will lose its brilliant color for a more tannish brown appearance and show white bite marks. The coloration of the Acropora alone is easy to dismiss. As we have discussed before, Acropora can change color dramatically for numerous reasons so often times a hobbyist will ignore a colony starting to brown out and attribute the change to some other changing parameter.
Once the white bite marks start to appear though it is clear that there are flatworms present and the coral must be treated with a series of pest control dips to remove the existing flatworms as well as newly hatched flatworms that emerge from the eggs that are resistant to dipping. In some extreme cases of flatworm infestation, the pest control dip will dislodge a shocking number of flatworms to the point that it looks like there were more flatworms on the coral than actual coral flesh.
Once Acropora colonies get huge or fused to the rock work, periodic dipping is no longer a viable solution. At that point whole-tank treatments must be considered such as flatworm exit or levamisole. Both of those treatments cause a lot of death and destruction in the tank so it is important to do massive water changes and running activated carbon aggressively to remove both the treatment and the toxins released from hordes of dying invertebrates. It is an extreme measure no doubt, but in a well-established tank sometimes that is the only alternative.
Typically I look for fish or some method of biological control of pests, but unfortunately I have not found anything that adequately deals with Acropora eating flat worms.
Red bugs are less of a concern than flatworms but should be removed none the less. They are essentially fleas on the coral that irritate the colony and might slow the overall growth of the coral or limit its coloration. The issue with red bugs is that they are very small and are difficult to see. Most hobbyists with Acropora struggling with red bugs never even know the pest is present. It’s only when another hobbyist points out the infestation that it becomes clear. Like with many things in this hobby once you see a problem, you can’t unsee it so when looking closely at any Acropora colony they immediately pop out.
Red bugs seem to be resistant to a wide range of dips however some of the more aggressive dips do seem to work. The dips based on pine oil tend to be too weak while dips like Bayer insect killer tend to be more effective. The best chemical treatment I have come across is a prescription medication for dogs called Interceptor that can be used to treat an entire tank.
The risk with more harsh chemicals is the coral itself might be aggravated by the dip and start to lose tissue. When dipping always consider the risk reward because some pests are relatively mild and the treatment could be far more damaging. My personal favorite way to eradicate red bugs is to add a fish called a dragon-face pipe fish. They are known to eat the red bugs and over time eliminate the problem.
The problem with dragon-face pipe fish is that they are not particularly good swimmers. They can handle swimming in the flow of an SPS aquarium however often times that flow is generated by large power heads sitting inside the aquarium. It is possible that some of these pumps are too strong and can kill the poor fish that get caught in its suction. What some aquarists have done is to keep a refugium that is plumbed into the main system and house the pipe fish in that tank. If an Acropora colony gets red bugs, it can be removed from the display tank and into the refugium for a period of time.
One last suggestion with pest control is to have some specific Acropora that are MORE prone to these pests in the tank. In essence they are acting as the proverbial canary in a coal mine. In large aquaculture facilities these corals are great not only for detection but also for treatment. By aggressively monitoring and dipping/physically removing the pests from these corals over time it eliminates the problem from the entire system. Clearly it is possible for pests to move off of these corals and onto others, but the pests have a preference for these “canaries” and over time find their way to them for extraction.
Summary
Ok, that about does it for Acropora. Although I cannot recommend Acropora for a beginning hobbyist, hopefully this video is helpful for those looking to try them for the first time. They are a beautiful coral and a tank dedicated to them can be a breathtaking explosion of color that rewards all the diligence on the part of the hobbyist.
