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Stability and Increasing Coral Growth in the Reef Aquarium

Updated: Nov 10, 2023

Coral stability and growth in the reef aquarium

Stable water parameters are paramount to coral health, beauty, and growth. Stability is only one of many important factors that contribute to these conditions. After the nitrogen cycle is complete, and all water parameters are within their safe ranges, keeping them stable within those ranges is probably the most important contributor to stress free, healthy, beautiful, and growing coral.

Obviously, light, flow, regular feedings, and space are equally as important in their own way. Those multifaceted factors will be discussed in a later article. Here and now, I’ll focus on the benefits of stable water parameters.

Why are My Corals Not Growing?

The reef aquarium can never be an exact replication of the reefs in the oceans. It's just not possible to perfectly replicate the near infinite elements of that massive ecosystem.

Instead, we should strive to make our home aquaria sustainable, artificial ecosystems that mimic the natural reefs as much as is beneficial.

The foundation on which we build that artificial ecosystem is stability of accurate water parameters. While we may never exactly match the light, nutrient flow, or grazing prowess of a natural reef, we can build a healthy system on one of the ocean’s most powerful and most easily reproducible characteristics, its stability.

There are a million reasons why the corals in our aquariums may not be doing as well as we would like. From disease to improper environment and macro-pests, the specific reason or reasons why some of your animals may be experiencing difficulty are too numerous to delve into in this article, but I'll provide a few examples.

  • Poor water condition

  • Parameters outside of safe ranges

  • Improper lighting

  • Improper Flow

  • Disease

  • Macro-pests (irritation)

  • Lack of Food

  • Aggression

While these examples can affect coral growth, my intention here is to discuss one of the major inhibitors of coral growth, unstable, or inconsistent water parameters.

What is Stability?

Stability is relative, even in this context. Some may consider a daily temperature fluctuation of two degrees Fahrenheit stable. Others classify anything outside of a half degree fluctuation as unacceptable.

While half a degree is technically more stable than two degrees, it is more difficult to achieve, thereby making it impractical. Two degrees in temperature is also too wide of a fluctuation and in most cases one degree is just as achievable and more stable.

My point is stability is not just about reducing the fluctuation. It's also about what is practical. If you can’t maintain a half degree stability with any consistency or reliability, then does it really qualify as stable?

Stability is keeping a parameter from fluctuating outside of a given range in a practical method that results in consistency.

Which Reef Aquarium Parameters are the Most Important to Keep Stable?

The most important parameters to keep stable in the reef aquarium are:

AOI Zooanthid polyps
  • Temperature

  • Salinity

  • pH

  • Alkalinity

  • Calcium

  • Magnesium

  • Lighting

  • Nutrients

  • Flow

Why Stability is Important for Increasing Coral Growth

Most ecosystems in the ocean are stable. They have the same pH, temperature, alkalinity, macro, and micro element levels. They experience the same lighting, nutrient, and flow regimes daily.

In the aquarium, we have seen many examples of the results of stability. Corals that have a stable environment don’t have to spend energy constantly adapting. This results in less stress, better health, immune systems, and ultimately faster growth.

There are factors that are not stable as I defined it earlier, such as nutrient flow. The amount of nutrients, the form it comes in, and exactly when it comes can vary daily. However, the fact that at some point nutrients surge through a reef multiple times a day, every day, does remain consistent.

When considering consistency and stability, sometimes we need to “zoom out” from a microscopic or minute by minute view to see how stability still applies, just on a larger scale.

For example, temperature should be maintained constantly. Coral feedings and nutrient flow, on the other hand, achieve stability by occurring at the same time daily, not every few minutes like the switching on and off of a heating element. Although this is relative to your aquarium. Systems with lots of fast growing corals can be fed multiple times a day with massive nutrient export systems.

Achieving Stability

Achieving stability is all about knowing the acceptable and practical range you want to keep your given parameter within, and implementing the proper methods and technology to achieve that.

The first aspect, knowing the acceptable and practical range, can be relative, like I mentioned earlier, but the key is to know what works with your tank. We’ll get into it with each parameter later, but some aquarists keep their alkalinity at 10, others at 7. Stability would not be keeping it between 7 and 10, but as close to whichever number you choose as is practically achievable.

The second aspect, implementation, is extremely nuanced with several viable methods, each with their own advantages and disadvantages. I won’t go into detail on each one. I’ll simply list them so you can explore each one in more depth at your leisure.

Playing the Long Game

Once you have achieved a consistent and stable range of parameters you are happy with, the rate of coral growth won’t happen overnight. It may still take several months before your corals adapt to stable conditions and begin to grow faster.

Don’t forget, stability is but one aspect of Increasing coral growth. Nutrient intake, lighting, flow, and many other factors play a large part in coral biology as well.

Stable Water Parameters in the Reef Aquarium


Keeping your temperature stable is probably easier than the other parameters, and just as important. You should strive to keep temperature fluctuation within 1 degree Fahrenheit. If this proves too difficult for your system, 1.5 degrees is still better than 1, but anything over 2 degrees of fluctuation can begin to stress more sensitive creatures.

When performing water changes, try to heat up the new water to the aquarium temperature, especially when performing a water change larger than 10%.

The easiest way to achieve temperature stability is by utilizing a controller for your heating element. Some heaters are efficient enough to keep temperatures stable using the built in thermostat. However, installing a heating element, attached to a separate controller with at least two temperature probes attached, is much more reliable and consistent.

Two temperature probes allow for a more accurate reading of the aquarium's actual temperature. The controller gives more precise control over the on/off reaction times of the element. Choosing a controller that allows multiple elements to be installed also guarantees a more significant, and therefore consistent heat distribution throughout the system.


Animals that live in aqueous environments are constantly undergoing osmosis and diffusion. This means, they are always balancing the concentration of water, and the solids dissolved in it, inside and outside of their tissues.

If the concentration of the major dissolved solids (in this case salt) in the water is constantly changing, then the animals become stressed by spending too much energy trying to correct these changes in concentration.

Additionally, water moves through the tissue via osmosis, which takes no metabolic energy, but can still significantly affect the concentration of other dissolved solids, causing stress on the animal.

However, if the salinity is always the same, the animals can spend their energy on their immune system, growth, and other metabolic functions.

There are many solids dissolved in seawater, but salinity is the measure of NaCl, sodium chloride, which makes up nearly 70% of seawater. Keeping salinity stable is far more important than alkalinity, calcium, magnesium, trace elements, and organics. Once your salinity is stable, then you can focus on the lesser concentrated solids in the water.

Why Would Salinity Change?

Salinity can change for many reasons. The most common and quickest to occur is evaporation of water from the tank, resulting in a rise in salinity. This is easy to offset by adding RODI, or non-salinized freshwater, back to the aquarium, at the same rate it evaporates.

This can be done manually, by pouring freshwater into the aquarium from a storage container, which is sealed when not in use, or as you make it. Marking where the top of the water level should always be in your aquarium, allows you to refill to that mark consistently as the tank water evaporates.

Another option is to install an automatic top off. This method, once installed, takes care of itself. Some versions still have a reservoir that needs to be refiled. Other versions hook directly up to the RODI system. The latter is not as safe as having a top off reservoir, because if the float valve, water sensor, and its backups fail, the RODI system will continue to feed water into the aquarium indefinitely. A reservoir will only dump the reservoir’s amount of water into the tank if it fails.

Salinity can also change when the new water mixed for water changes is not accurate. When performing water changes, always recalibrate the tool you use to measure salinity. Measure the salinity of the aquarium and the new water. If your tank reads high, lower the salinity in the new water and vice versa.

For example, if your tank is at 36 ppt and you want it at 35 ppt, and you are doing a 10% water change, then the new water needs to be at 26 ppt. If you're doing a 50% water change, then the new water should be mixed at 34 ppt to bring your tank to 35 ppt.

That's just the math though. It is more practical and safer to adjust salinity slowly over several water changes.

Another common way salinity can change is if you are dosing sodium bicarbonate or sodium carbonate with calcium chloride. As the animals use the bicarbonates or carbonates and the calcium, the sodium and the chloride combine into sodium chloride (salt), and raise the salinity.

There are several ways to go about correcting this, the most common is the method from above. Measure the salinity of your aquarium before your water changes and mix the new water to the correct salinity to bring your aquarium back down to 35 ppt.

If your salinity is climbing because you're dosing these two compounds, be sure to research the “Balling Method” for 2 part dosing. There is some interesting chemistry behind what happens to your trace elements and magnesium levels when you are 2 part dosing calcium chloride and carbonates and then correcting for the salinity change. If you don’t add trace elements back into the aquarium, then you will reduce them indefinitely.


A stable pH helps stony corals maintain a strong, dense skeleton. There have been decades of multidisciplinary research and studies on pH stability in the ocean. pH is a standard metric used to measure ocean stability.

In the reef aquarium, we are coming to realize maintaining a pH range of 7.8 to 8.3 throughout a 24 hour period is just the beginning. While 7.8 to 8.3 is the safe range, closing that gap to 8.25 plus or minus a tenth of a degree, or better yet, 8.3 for 24 hours, is exponentially better for coral health and growth.

There is still much research left to be done concerning the relationship with pH and coral health in the reef aquarium. Although, what we have discovered recently suggests stable pH as close to 8.3 as possible has a much larger affect on coral health than we thought.

pH stability is its own article. However, there are some easy ways to close the gap, all else being equal.

First, if you have a refugium or plan to set one up, the photoperiod on your light should be the opposite of the display tank's light. Photosynthesizing macroalgae will absorb CO2 at night, preventing the pH from dropping significantly.

Another common cause of low pH or pH instability is too much CO2 in the air around the tank. Running airline from the skimmer to the outside or opening a window can bring CO2 levels down.

Stable alkalinity can also help with maintaining a stable pH.


The capacity by which a body of water can maintain a stable pH through the neutralization of acids and bases is what we are measuring when we test for alkalinity.

For most of us with a reef tank, this means the concentration of carbonates and bicarbonates in the water.

Corals and other microfauna utilize carbonates as part of their biology. When they absorb carbonates from the water, their concentration lowers, thus lowering the alkalinity. Alkalinity can also lower when carbonates are combined into other compounds, such as with CO2 gas.

The alkalinity is replenished with water changes and dosing carbonates and bicarbonates directly.

The key to stable alkalinity is not to just replenish carbonates weekly or every couple weeks in one huge dose, like with a water change, but to add them at the same rate they are used as often as possible.

Most of the aquariums I maintain receive a dose of sodium bicarbonates multiple times a day; usually a minimum of two. Aquariums with a relatively high population of stony corals can drop in alkalinity by as much as two or more degrees of carbonate hardness (dKH) in a week. That's nearly 0.3 degrees a day. By adding the appropriate amount of sodium bicarbonate to raise the alkalinity of the tank by 0.15 degrees twice a day, the most the tank will fluctuate is .15 degrees instead of two degrees. In terms of maintaining stability for coral health and growth this is an incredible difference.

The easiest way to dose carbonates for consistent alkalinity is to use a dosing pump. These range from simple single headed pumps that can plug into mechanical or digital outlet timers to highly programmable multi-pump dosers.

Whether you choose a simple single-headed doser, or a programmable apparatus with a built in computer, they are both means to the same end, stability.

Calcium & Magnesium

Without getting too much into the nuance of the chemical relationship between alkalinity, calcium and magnesium, the main take-away is calcium and magnesium can be dosed similarly to alkalinity.

Corals utilize calcium and magnesium just like carbonates. The more often you replenish calcium and magnesium, the more consistent their concentrations will be.

There are a couple things to consider when dosing these elements with carbonates. Briefly, I will mention that when dosing alkalinity and calcium, they should not be dosed at the exact same time. Also, the Balling Method, mentioned in the salinity section above, also applies here as the rise in salinity mentioned is a result of dosing sodium bicarbonate or sodium carbonate with calcium chloride and results in a trace element deficit over time.


Recently, we have begun to discover that corals are capable of adapting the types and concentrations of their light sensitive pigments and chlorophyll. Even though corals are highly adaptive, this process is biologically expensive. Even if your lighting is not ideal, your corals may compensate over time.

However, setting your lights to an appropriate intensity and spectrum for the livestock in your tank, and not altering it after, is paramount to stability.

Use a trusted PAR meter and the spectrum information, or available settings, on your light to determine what is the best scheme for your setup. Find a fellow hobbyist with a similar and successful tank to yours with a working lighting scheme, or use the presets in the app if your light comes with one.

Intensity and spectrum are both important. Nothing beats using a PAR meter to measure intensity. Spectrum is a little more difficult. All we really have is what the side of the box says or the adjustments available in the app if your light has one. The details on exactly what spectrum to favor is for another article. Suffice it to say, corals prefer a higher intensity around 420 nanometers (blue), but still require a lower intensity across the spectrum (white).

Minor adjustments over the course of the life of the tank, even larger ones at the beginning are fine. The constant altering of your light's settings on a weekly or even monthly basis are what leads to instability and coral stress.

Remember, even if your lights are not ideal, your corals will adapt. It is adapting constantly from frequent changes that can be detrimental to coral health and growth.

Nutrients and Flow

Stable nutrients (organic particulate matter) and flow work a little differently than the above parameters. Consistency in schedule is more important than constant stability. Unlike the parameters above, nutrients and flow should fluctuate over the course of a day.

Stability is achieved by feeding the same time every day and by having the same flow patterns at the same time every day.

For example, with flow, you may have a pulsating pattern at 20% for most of the day, then a feed mode at 7 pm, then a high nutrient pulse for 2 hours at midnight. Stability in flow means keeping this scheme or schedule of patterns the same every day.

In the ocean, flow changes throughout the day, but the intensity and pattern is fairly consistent at the same times. The same is true for nutrients.

Both fish and coral can anticipate feedings by the “time of day” or the light intensity if you feed at the same time everyday. Coral will extend feeding tentacles and are more receptive to incoming food particles.

Reef Aquarium stability and healthy corals in Kansas City

Slow and Steady

Maintaining stability and consistency in water parameters is crucial to coral health, growth, and beauty. While achieving stability is important, any changes made in a reef tank must be done slowly. Unless it is an emergency, like extremely high temperature, salinity, or pH, altering your parameters to the range of numbers you want, then keeping them stable within that range should take time.

Aquatic life is particularly subject to the environment it lives in. Changes in the concentration of elements and compounds in the water quickly and significantly impact the biology of aquatic organisms. Making these changes slowly, even if they are for the better, is important to reducing stress on the organisms.

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