What Makes Coral Grow?

What Actually Makes Coral Grow?

Although corals vary widely by species and color, all corals generally thrive best under specific conditions based on their natural habitats and growth processes. Everything from Candy Canes to Kenya Trees will come with natural preferences for parameters like water stability, lighting intensity, and nutrient exchange, and you’ll notice visible signs if your coral is happy. These preferences vary almost as much as the corals themselves do, but understanding what makes coral grow can help demystify the process of caring for a reef aquarium.

Coral Growth Starts with Stability

Many of us are familiar with the importance of stability when it comes to coral growth, particularly if your tank is on the smaller side. One of our first experiences with reefing was with a 10 gallon nano tank, which immediately reinforced the importance of proper maintenance routines. It’s important to remember that water stability is heavily dependent on tank volume, meaning that larger tanks allow for a lower frequency of water changes and dosing, and are less susceptible to sudden imbalances in water chemistry from feeding.

This concept is especially important for coral growth, which requires consistent stability and patience. Unlike many other reef animals, visible reactions may take minutes to hours to manifest when a coral reacts to a change in its environment. While relatively hardy, corals are also fragile and very dramatic about chemical and environmental changes. Water parameters like pH, alkalinity, temperature, and more must be in healthy ranges and remain stable over time for corals to achieve optimal growth rates. The ideal ranges of these vary a bit from coral to coral, but soft corals and stony corals often share similarities with others of their type.

This table lists the general ideal ranges for the factors that affect coral growth, but please research your specific coral type to confirm how you should care for your reef. These parameter values are intentionally listed as ranges, because stability is far more important than chasing specific numbers. It’s also beneficial to have trace amounts of strontium, potassium, and iron, which are pre-mixed in some marine salt mixes with varying ratios.

Light and Flow: The Energy Drivers

The table above may look equal parts helpful and overwhelming, but these numbers are achievable if we move with intention and focus on stability instead of exact values. This holds true for lighting and flow as well.

Lighting

Light provides energy to coral through photosynthesis, so it’s important to buy reef quality lights that have enough power to cover your aquarium size. While many corals benefit from being fed, photosynthesis is an essential part of natural coral health. Corals host small organisms called zooxanthellae, which are actually a symbiotic algae that drives coral growth through photosynthesis. This process produces sugars that fuel the calcification of the skeleton. Lighting is not a piece of equipment to cut corners on if you want to maximize coral growth, as many imitation reef lights lack some of the color channels that promote photosynthesis and aesthetics. Specific color wavelengths like blue, white, and red affect coral health, with blue being the primary driver of photosynthesis. Red and green wavelengths help enhance coloration, so it’s beneficial to have a light made for marine aquariums.

Regardless of the quality of your light system, it’s important to make lighting adjustments gradually, especially when introducing new coral to your system. Some corals prefer to have dimmer lights when first added to the ecosystem, when moved, or when other factors in the environment change. Make sure to conduct research on your specific coral(s) to create the most beneficial ecosystem you can.

Light cycles for corals range from source to source and species to species, but most corals prefer to have reef lights on for between 8 and 11 hours per day. It’s also recommended to begin and end the light cycle with a 30 to 60 minute duration of lights at 50% of the full daylight power setting. In terms of general preference, softies enjoy lower PAR ranges from approximately 50 to 150, LPS thrive in moderate ranges from roughly 75 to 200 PAR, and SPS prefer high PAR ranging from approximately 200 to 400. What makes coral grow optimally in your reef tank will be dependent on the specific kind of coral you have, so always conduct light research on your individual corals.

Flow

Flow, on the other hand, helps coral receive and absorb nutrients from the water, and prevents imbalances resulting in detritus and other problems. Reef aquariums generally need a consistent, moderate flow to keep the water circulating through the filtration system and facilitate gas exchange. Achieving the optimal flow pattern for your tank that makes the entire ecosystem happy can be a little tricky, however.

In general, you want to try to create a calm circulating pattern that runs throughout your tank continuously. Most corals don’t enjoy being blasted by direct flow any more than they want to get neglected in a dead water zone. Coral benefit from water movement, not from direct current to the face. Many softies prefer a low to moderate flow for ideal growth, while many LPS prefer indirect turbulent flow, and many SPS prefer moderate to strong flow. With both flow and lighting, it’s much easier to start on the lower side of intensity and wait an hour to see how your coral reacts to the conditions. Slowly increasing the flow until you’ve reached the recommended flow pattern for your coral is gentler on the coral than starting at the desired intensity and making the coral acclimate to it directly.

Nutrients: The Balance Between Starving and Overfeeding

As mentioned previously, many corals benefit from being fed things like pellets, phytoplankton, mysis shrimp, and other feeds. Heterotrophic (manual) feeding produces noticeable benefits for growth, especially in many LPS corals, so finding a balance between feeding and photosynthesis can produce amazing results. However, it’s also possible to overfeed your coral and cause more harm than good.

One of the things you’ll become intimately familiar with as a reef bender is the delicate balance of nutrients. Everything from calcium (Ca) to nitrates (NO₃⁻) matters more than it feels like it should, and nutrients are always being imported to and exported from your tank to an extent. The aeration of your filtration system allows for carbon dioxide to release from the tank, which helps the pH stay stable. The magnesium (Mg) in the water prevents the calcium from interacting with the alkalinity and causing precipitation. You do, in fact, have a small ecosystem in your aquarium. Most fish and other types of animal foods naturally break down into ammonia and phosphates in your tank when left uneaten. The ammonia is converted into nitrites and then nitrates by the live culture of microorganisms in your tank. Certain macroalgaes like chaetomorpha and dragon’s breath help to export these nutrients to keep levels stable in your tank, but eventually water changes must be performed to keep nitrate levels low.

Overfeeding results in additional ammonia and phosphates entering your tank, which can be irritating or harmful to animals until they are converted to nitrites and nitrates. Even then, nitrates are still toxic for many corals at higher levels like 30 ppm or more. It’s important to try to only feed as much as your animals can comfortably eat, and no more frequently than is recommended for your specific coral. Additionally, water changes should be performed roughly every two weeks for most tanks regardless of feeding practices, and should replace 10 to 20% of total water volume. If combating issues with nuisance algae/bacteria, feeding less, increasing or adjusting flow, or slightly shortening lights-on time may be helpful things to explore.

The Role of Substrate in Coral Growth

One of the most overlooked pieces of what makes coral grow is the substrate upon which it grows. The substrate noticeably affects the rate of encrustation, especially for those that encrust outward rather than upward. Because most corals are immobile, being attached to an appropriate substrate makes a big difference for proper growth long term. The main factors that affect substrate quality are surface texture, microbiome colonization, calcium availability, and attachment energy.

Surface Texture

Porosity and surface texture help the coral actually attach securely to the substrate, which helps them grow with stability. Many corals will show slow or stunted growth if not allowed to attach to a secure substrate, which can be a result of low porosity or smooth surface texture. Proper surface texture encourages steadier encrustation over time, because firm attachment means that as the coral grows and its weight distribution changes it does not need to re-establish its grip repeatedly. In addition to facilitating stable coral growth, porosity and texture play a role in the health of the microbiome as well.

Microbiome

The microbiome is the collective group of microorganisms that make up the foundation of your “clean up crew”. As the foundation of your clean up crew, the microbiome is a community of bacteria, microfauna, copepods, tiny isopods, and other microorganisms that consume uneaten food and other waste in your tank. Waste breaks down into ammonia, which nitrifying bacteria convert into nitrites and then nitrates. These nitrates are less harmful to your tank and can be managed with macroalgae, so the microbiome is your unseen work force for tank stability. Surface texture, porosity, and substrate material affect the development and health of your microbiome, and in turn your reef ecosystem. This means that choosing the right dry rock, sand, and frag materials will produce huge benefits in the long run.

Calcium Availability

Calcium availability refers to the composition of the substrate, which can actually aid in coral growth. To form the initial attachment to a substrate, corals secrete an organic film on the surface and deposit a calcium carbonate skeleton. Coral create these skeletons by combining dissolved calcium and carbonate ions, so media that helps to mildly buffer calcium intake as it’s consumed can aid coral growth over time. Dry rock is often rich in aragonite, and many sands have crushed coral mixed into grains of aragonite or other calcium-carbonate media. Additionally, some frag discs and frag plugs are made with calcium-carbonate media to assist with buffering and utilize a natural substrate. In terms of overall coral growth, stable alkalinity is more important than specific calcium levels.

Attachment Energy

Attachment energy is directly affected by porosity and surface texture. This concept refers to the physical and metabolic effort the coral has to exert to attach to the substrate. Surfaces with minimal texture require more time for the coral to deposit and calcify a skeleton upon. Consequently, any time the coral is focused on securing attachment is time that the coral is not exerting its energy towards growing. This means that surfaces like glass, smooth ceramic, or non-porous rock will slow skeletal deposition and tissue adhesion. On the reverse, reef rock, texturized ceramic, or porous calcium-carbonate media will allow encrustation to progress without delays.

Making sure the substrate your coral grows on is suitable for long-term encrustation is one of the most important steps, but there are some common options that work well. Calcium-carbonate substrates like rock and aragonite blends are popular and offer great advantages, and in many cases you can visually assess surface texture to confirm it’s not too smooth.

Why Micro-Environment Matters More Than Most Reefers Realize

It’s easy to run tests on your water chemistry or lighting to get generalized readings for your aquarium, but it’s also important to consider the quality of the micro-environment your coral is growing in. Factors like localized chemistry, biofilms, micro-flow zones, and surface chemistry affect the growth of attached corals even if they don’t produce measurable effects in the tank. It’s important to include these elements of the micro-environment when considering what makes coral grow.

Localized Chemistry

Corals are affected by both the ambient chemistry of the water and the localized chemistry directly around their bodies. Things like live rock, sand, macroalgae, other corals, and reef frag plugs can all have an impact on the localized chemistry coral is exposed to. For example, live rock can serve as a calcium buffer and provide substrate for the microbiome that helps process waste, or they can serve as breeding grounds for bacteria if nutrients are not balanced. Closed holes that trap food can cause localized spikes in phosphate if not cleaned manually or by animals, so there are many ways that localized chemistry can shift while the overall water chemistry remains relatively stable.

Biofilms

Biofilms are communities of microbes that are formed on virtually every surface in your tank, from your sandbed to your water heater. These biofilms are an essential part of the reef ecosystem because they process the waste of the tank and support a healthy microbiome. Biofilms provide the foundation for nitrifying bacteria, which process ammonia into nitrites and then nitrates. However, as with most parts of the reef, imbalances in the biofilms can facilitate the growth of nuisance bacteria and algae. Proper aquarium maintenance and sustainable media are crucial for long-term reef health and optimal coral growth. This means that known traps in your dry rock must be cleaned regularly, change your water on a schedule, and choose structures and frag plugs with appropriate porosity. Product porosity varies widely among frag plug manufacturers.

Micro-Flow Zones

Micro-flow zones describe the tiny eddies and currents that sometimes swirl through specific sections of your aquarium in semi-regular patterns. While your major flow pattern refers to the general circulation of water throughout your tank, micro-flow zones are smaller sections that have noticeable flow patterns that deviate from the major flow. Micro-flow zones can provide additional nutrient absorption for softies, or they can silently peel the polyps off a hammer coral in a day. It’s important to really study the way the water flows throughout your reef aquarium beyond just the direction it’s going. Look at the channels between rockwork, the bubbles on the surface, the sway of polyps over the course of a few minutes, and the extension of the polyps.

Surface Chemistry

It’s essential to know what material your coral is growing on so you know how it’s affecting your coral’s local water chemistry. Some materials like ceramic are chemically inert, so they won’t have any effect on the water chemistry of your tank. Other materials like aragonite are calcium-based and encourage faster encrustation. Still other materials are bound with chemicals that can be harmful to the tank if not properly cured. A lot of the options mentioned above are viable substrates, but they each impact local water chemistry differently, so being knowledgeable about what you have in your tank is important.

Bringing It All Together

With all of these different factors spelled out for what makes coral grow, it can be easy to compare them to gremlins at times. Although I wouldn’t recommend it, I can assure you they don’t turn evil if you feed them after midnight. They’re just sensitive.

Coral growth is a multi-factor process that requires an intimate understanding of your materials, your chosen parameters, and the biology at play in your reef. Everything from the height at which your coral is placed to the type of substrate on which your coral grows can enhance growth by supporting stability or hinder encrustation by supporting harmful microorganisms.

If you notice your coral is bleaching or color loss, tissue recession, or polyp retraction, it may be time to reassess the elements of your tank and the parameters of your coral’s environment. Bleaching is caused by the expulsion of zooxanthellae during stress, so your coral will often give you signs if you pay attention. Even with impeccable maintenance habits and beautifully engineered aquascape, if your coral is battling localized water chemistry it may never truly thrive. Each element of your tank plays a role in the support or deterioration of stability, so place as much thought into the environment that surrounds your coral as you do into the corals themselves.

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