I’ll admit that when I started looking into reef tanks, I spent countless hours looking at pictures of gorgeous coral reefs completely enamored. “Coral trees? Brain coral? These things look like plants!”
And some of them are!
Except when they’re not. You add some macroalgae for nutrient export and suddenly you’re questioning what you can legitimately consider a plant vs animal in your saltwater aquarium.
But wait, there’s more!
As you go down the rabbit hole you’ll discover animals that look like plants, plants that look like seaweed, algae that does more heavy lifting for water quality than anything else in the tank, let alone coral vs plant care requirements.
So what *can* you keep in your reef tank and how do you know what you’re actually keeping?
There are real biological implications to understanding the difference between coral and plant life for your aquarium. Feeding, lighting requirements, water chemistry and long-term stability of your system all depend on understanding the organisms you choose to add to your saltwater aquarium.
Equipment selection, maintenance tasks, cost and even aesthetics are impacted by whether you want a rock full of coral or an island of macroalgae.
## Let’s Get Scientific
Ok, so corals *are* animals. But why does that matter? Let’s dig into the details.
Corals are colonial animals in the phylum Cnidaria that have tentacles surrounding mouths (Britannica). A coral head is made up of thousands of tiny individual animals called polyps. These individual animals cluster together to form a coral head and build a shared calcium carbonate skeleton.
_Coral Classification_ – NOAA also classifies all coral species as animals in the phylum Cnidaria (NOAA Coral Disease and Health Consortium). This includes all the jellyfish freaks of nature you may have seen at the zoo as well as sea anemones. Within reef keeping, you’ll generally see coral broken into two classifications – Hexacorallia and Octocorallia. Hex have polyps with tentacles in multiples of six, while Oct have….you guessed it, eight tentacles (University of Louisiana at Lafayette). There are around 3,500 known species of Octocorallia alone (World Register of Marine Species), which includes soft corals, sea fans, and organisms commonly referred to as gorgonians.
_Coral Food & Light_ – Because corals are animals with mouths, they require food. Corals require dissolved organic compounds, amino acids, and small particulate food matter. Corals also rely on photosynthesis from algae that lives symbiotically within their tissues known as zooxanthellae. Zooxanthellae provide nutrients to their coral host by photosynthesising and sharing that energy. This is why proper lighting is important to the health of your coral but your coral is not technically photosynthesising.
_Real Plants_ – Flowering plants in saltwater, also known as true marine plants, are surprisingly rare. Seagrasses are one of the few categories of true marine plants found in full marine environments (Aquanswers). These have all the parts of land plants we’re familiar with – roots, leaves, flowers, etc. They’ve simply evolved methods of exchanging water through their roots to overcome osmotic pressures of living in saltwater and can flower underwater.
_Macroalgae_ – These large, multicellular algae look and act like plants but lack true roots, stems and leaves. Most macroalgae you’ll find sold under “marine plants” at your LFS fall into this category. Think species like Chaetomorpha, Caulerpa, and Halimeda.
Why Should I Care?
Pretty soon you’re going to have to pick equipment based on whether you’re keeping animals that need to eat or macroalgae that devours nutrients. What equipment you choose, how you run your maintenance schedule, what livestock you can successfully keep… it all depends on how you answer the coral vs plant question.
_Coral Requires_a_diffent_type_of_care than macroalgae. They may both make your reef tank look full and lush, but that’s about where their similarities end. Miss this distinction and you’ll either starve your livestock or nutrient burn your tank trying to get it right.
The Lighting
Coral require powerful lights that can penetrate the depth of your aquarium. You’ll want something that can put out light in the 400-700 nanometre range with peaks in blue and white lighting. Most successful reef keepers run some type of LED lighting technology. Lighting for macroalgae, on the other hand can get away with more basic requirements. Macroalgae doesn’t have to photosynthesize through an animal host and can survive (and often thrive) on minimal lighting. This is ideal for refugiums.
Water Movement
Water movement is important to keep detritus from building up on macroalgae. Most species prefer moderate flow. If the flow is too strong it can damage macroalgae or tear it from its ties. Corals have specific flow requirements based on species. Soft corals prefer random weak flow. Some hard corals prefer moderate flow. SPS corals want strong flow.
Substrate
Hard coral skeletons are made of calcium carbonate. Maintaining calcium and alkalinity levels are important. Substrate can play a role in the buffering capacity of your tank. Aragonite sand buffers top-up water and helps maintain stable alkalinity levels. Live rock also helps buffer water but it’s main contribution to algae gardening is providing bacteria surface area and livestock attachment points. Some macroalgae require a place to anchor to while others float freely.
Nutrients
Remember how we talked about corals being animals? While some dissolved nutrients are ok they will react poorly to elevated nutrient levels. Macroalgae, as pretend plants, will consume nutrients as they grow. Studies have shown that macroalgae will readily uptake ammonium, nitrate, and phosphate from the water column they occupy (ScienceDirect).
Feeding
Coral will require you to feed your livestock. Not just any food either. Depending on coral type you’ll be purchasing things like zooplankton, phytoplankton, amino acids and/or look for dissolved organic compounds. Some corals, like many large polyp stony corals, rely on direct feeding. Others, like small polyp stony corals rely mainly on their zooxanthellae. Macroalgae harvest nutrients from water column. There is no supplemental feeding required.
Tank Integration
Many reef keepers choose to run refugiums, completely separate from their display tank, specifically for keeping macroalgae. This allows you to tune conditions to what macroalgae need without compromising your coral display. It also makes harvesting algae easier without stirring up your display tank. If you choose to keep macroalgae in your display tank, there are many options that can compliment your livestock. BuildYourAquarium has a great list of 10 Macroalgae Species to Keep in Your Reef Tank (BuildYourAquarium). Options range from plain old Chaetomorpha for nutrient export to decorative options like Halimeda or Caulerpa.
## 5 Things New Reef Keepers Do Wrong
You know those people that build amazing reef tanks in 6 months? Yeah, they didn’t get there without making some mistakes along the way. The problem is, some of these mistakes new aquarium enthusiasts make can kill a reef tank before it ever gets started. Understanding the difference between coral and plants can help you avoid some common pitfalls.
Number 1: Thinking corals are plants. This almost goes without saying but new hobbyists often jump right into reef keeping because they grew up having success keeping planted aquariums. Corals are animals. They require food, stable water parameters and proper water movement. Just because they live submerged in water doesn’t mean they can go without feeding like a freshwater rubber plant.
Number 2: Forgetting macroalgae require nutrients too. Macroalgae are often added to aquariums with the express purpose of exporting nutrients. What’s forgotten is that macroalgae are still alive and require nutrients to grow. Plant them in an extremely low nutrient system and they’ll die off. When they die, they decompose and release nutrients back into the system. The exact opposite of what you wanted.
Number 3: Picking coral and macroalgae that won’t get along. Harsh as it may sound, some macroalgae will simply overgrow corals. Stunted growth. Dead spots. I’ve seen it happen. Equally troublesome are corals that produce substances known as allelopathic compounds. Macroalgae and other aquarium plants are susceptible to these chemicals and you may find your algae failing to thrive because of your coral pick.
Number 4: Ignoring the chemical warfare happening in your aquarium. Chemical warfare. It’s real and it’s happening in your aquarium right now. Corals will release toxins into the water column to kill things that venture too close. Planted tanks live in harmony because plants don’t fight back. Throw some plants in your reef tank and your next water change could be surprisingly gruesome.
Number 5: Setting up your refugium wrong. I can’t tell you how many times I’ve seen hobbyists set up macroalgae refugiums without learning about how different species of algae grow and how often they should be harvested. These refugiums turn into nutrient bombs that swing parameters out of control and either kill your coral or promote ugly macroalgae growth in your display.
Want to learn more about how reef tank biology works? Cheque out this paper on Marine Ecology that dives into how much we still don’t know about coral nutrition and nitrate uptake (Springer). Macroalgae nutrient uptake, however, has been studied quite extensively.
Nutrient uptake is cumulative, meaning macroalgae pull nutrients from the water column as they grow. Control study on macroalgae nutrient uptake have been able to document specific ammonium, nitrate, combined nitrogen sources, and phosphate uptake rates (ScienceDirect). These results help aquarium hobbyists apply scientific findings to refugium design and nutrient export.
Are you keeping coral or macroalgae in your reef tank? Both.
## Mixed Reef Systems
Saltwater aquariums that keep hard coral, soft corals, and macroalgae will have to balance water parameters to meet the needs of both coral and algae. Moderate nutrient levels will help feed coral, but provide enough nutrients for macroalgae to flourish. Not all corals and macroalgae play nice together, though. Research your species before introducing them to the same tank.
SPS-Dominant Systems
SPS tanks will most likely run very low nutrients. Macro algae will still require some nutrients to survive, so choose species that can handle low nutrient environments. This usually means your options are limited to more efficient algae like Chaetomorpha.
Soft Coral Systems
These systems can handle higher nutrient levels. Many soft coral tanks integrate macroalgae right in the display tank. Soft corals are generally less aggressive than hard coral and many don’t produce allelopathic compounds.
Refugium Grow-Out Systems
Keeping macroalgae in a refugium gives you the ability to tune conditions to what your macroalgae needs. You won’t have to compromise your coral display to keep algae. This is also beneficial because it maximizes nutrient export and gives you the most aesthetic options for your display tank.
Nano Reef Aquariums
Nano reef tanks don’t have the luxury of large volumes of water to buffer mistakes. Stocking your tank too heavily with animals will create nutrient spikes macroalgae can’t keep up with. Stocking your tank with too much macroalgae can lead to starvation issues if the algae outcompetes your filtration for food.
## Benefits of Knowing the Difference
Understanding the differences between coral and plant life for your aquarium can have some distinct advantages.
Healthy Coral – One of the benefits of knowing your coral are animals is that you feed them. You watch them get hungry. Much like a fish would. If you feed them properly you’ll begin to see better polyp extension, brighter colours, and faster growth.
Effective Nutrient Export – Macroalgae can become one of the most useful filtration components in your system. When maintained properly macroalgae can significantly reduce both your nitrate and phosphate levels. Many hobbyists will notice water quality improvements in as little as 4-6 weeks of adding macroalgae to their system.
Appropriate Equipment – Lighting, flow pumps, and filtration can be tweaked to suit the biology you choose to keep. Keeping coral may mean spending more money on a protein skimmer. It also may mean your fishtank requires a calcium reactor. Macroalgae keeps leans more towards tuning refugium lighting and managing flow to keep debris from building up on your macroalgae.
Less Maintenance – When you understand how biological filtration works and what your organisms living in your aquarium need to stay happy you’ll prevent a lot of common maintenance issues. Dead coral from starvation. Macroalgae die off from lack of nutrients. Algae being eaten by fish. Coral poisoning your aquarium plants. These can all be avoided by planning ahead and understanding coral vs algae needs.
Saving Money – Want to add that new sexy coral you saw at the LFS? If you understand how much your livestock eats and how much your current stockproduces you can cheaply cycle your aquarium and avoid buying expensive livestock that won’t survive.
Stable Aquarium – When you stock your aquarium with compatible organisms that balance each other out you’ll enjoy more stable nutrient cycles and spend less time tweaking your aquarium.
A successful reef tank takes knowledge, planning, and patience. Building a reef tank that incorporates both animal (coral) and plant-like (macroalgae) organisms takes a little something extra. Understanding how these two biological filters work together will ensure you don’t add coral because it looked cool, and then watch it die because your aquarium was designed with algae in mind.
## Phase Implementation for New Aquariums
Truth be told, there is no “right” time to add macroalgae to your saltwater aquarium. That being said, if you want to add both coral and macroalgae to your tank there are some tricks to getting it right.
_Phase 1 – Establish your system (1-8 weeks)_
Cycle your tank and get comfortable with basic water chemistry. Add your reef lighting if you haven’t already and get basic flow established. Add hardy macroalgae like Chaetomorpha to begin processing nutrients.
Estimated Costs:
* Basic Reef Lighting: £150-400
* Protein Skimmer: £100-300
* Live Rock: £8-15/per kg
* Macroalgae: £20-40
Total: £278-755
_Phase 2 – Add Coral (9-16 weeks)_
Now you can add your first coral colonies. Pick hardy species that can handle the lighting and flow you can presently provide. Begin your coral feeding regiment and see how your macroalgae reacts to the nutrient export. Bring testing for calcium, alkalinity, and magnesium levels if keeping hard corals.
Estimated Costs:
* Starter Coral Colonies: £40-120
* Coral Foods/Supplements: £30-60
* Water Testing Kits: £40-80
Total: £110-260
_Phase 3 – Let it Rip (8-16 weeks)_
By now you should have a good understanding of how much food your coral requires and how much your macroalgae can process. Add additional livestock slowly and watch for compatibility issues. You should also establish a macroalgae harvesting routine to prevent unwanted nutrient swings.
_Phase 4 – Go crazy, you deserve it! (6-12 months)_
Start adding more demanding species now that you have established your system will run. You may even want to consider adding a refugium system at this stage to really crank up your macroalgae cultivation. Play with different feeding schedules and nutrient export techniques.
_Phase 5 – Maintain what you’ve built. (Year 2+)_
By now you should have stable maintenance routines that account for both your coral animals and macroalgae plant matter. Plan out your coral growth and possible propagation. Make sure you maintain genetic diversity with your macroalgae populations. Genetically similar algae die offs can devastate a refugium.
The most important takeaway I got when I started researching reef tanks vs saltwater aquariums was they’re not underwater gardens full of plants. They’re mixed ecosystems full of animals and plants pretending to be something they’re not.
Understanding coral and algae biology will not only help you keep a healthy reef tank, but it’ll save you money and help your livestock thrive. If you’re just getting started with reef tanks, make sure you cheque out our complete Beginners Guide to Saltwater Aquariums.
