I’ve been a believer in keeping a healthy fish population in reef tanks for years. Back then, when someone asked why their corals looked pale or lacked color, the advice was often simple: “Feed more” or “Add more fish.” It wasn’t based on scientific studies at the time, but years of hobbyist experience consistently suggested that a healthy, well-fed fish population helped improve coral coloration. Today, we spend so much time testing nitrate and phosphate, dosing amino acids, and experimenting with supplements that we sometimes overlook one of the most basic contributors to coral health: the fish we keep.
On natural coral reefs, a healthy fish population is one of the foundations of a thriving ecosystem. Fish do much more than swim around and look pretty. They’re constantly recycling nutrients through feeding and waste, supplying nitrogen, phosphorus, and other nutrients that help sustain corals and other reef life. Research has shown that removing fish from reefs can dramatically reduce this natural nutrient recycling. It’s a relationship that has existed for millions of years, and one that deserves more attention in our own aquariums.
As usual, let me preface this by saying I’m not a scientist. This is my own hobby-grade interpretation of published research combined with years of personal experience keeping and growing corals. The studies don’t answer every question, and results can vary by species and environment. But I think there’s enough evidence here to make us reconsider how we look at fish, feeding, and nutrient supplementation in our reef tanks.
Fish Waste Is More Than Just Waste
When we think about fish waste, most of us consider it something that needs to be removed before it breaks down and pollutes the water. But fish waste is also a source of nutrition. Fish release waste in two main ways. The first is through their gills and urine. Much of the nitrogen they release enters the water as ammonia and ammonium. The second is through feces, which contain undigested food, organic matter, nitrogen, phosphorus, and other nutrients that weren’t absorbed during digestion.
A 2023 study published in Oikos, The Role of Fish Feces for Nutrient Cycling on Coral Reefs, examined nutrient recycling across 51 reef fish species. The researchers found that feces can represent a substantial source of nitrogen and phosphorus, sometimes exceeding the nutrients released through dissolved excretion.
But another study published that same year revealed something even more interesting. Researchers analyzed fish feces and found not just proteins, carbohydrates, and fats, but also calcium, copper, iron, magnesium, manganese, and zinc.
These are many of the same elements we regularly test for and supplement in our reef tanks.
So fish waste isn’t simply something that eventually becomes nitrate and phosphate. It’s a mixture of organic nutrients and minerals that can contribute to the reef’s nutrient cycle.
That doesn’t mean we should allow waste to accumulate in our tanks. Excess organic waste can create problems, especially when it settles in areas with poor flow. But we shouldn’t look at fish waste as simply pollution either.
Fish Are Constantly Recycling Nitrogen and Phosphorus
In my previous article, How Corals Build Tissue and Color, I explained why nitrogen and phosphorus are essential to coral health. Nitrogen is needed to build proteins, including the fluorescent proteins responsible for many of the colors we see in corals. Phosphorus is needed for cell membranes, DNA, and ATP, the energy currency of living cells.
Without these nutrients, corals can have plenty of light and energy from photosynthesis but still lack the materials needed to build healthy tissue.
When fish digest food, they release nitrogen into the water, much of it as ammonium. Corals and their symbiotic algae can use ammonium directly. In fact, ammonium generally requires less energy to assimilate than nitrate because it doesn’t first need to be reduced.
Fish also release phosphorus through dissolved waste and feces. As organic waste breaks down, some of that phosphorus becomes available to corals, algae, and other organisms.
A 2016 study published in Nature Communications, Fishing Down Nutrients on Coral Reefs, demonstrated how fish populations contribute to nutrient recycling on natural reefs. The researchers found that fishing substantially reduced the amount of nitrogen and phosphorus being recycled by reef fish. Think about that for a moment. Fish are constantly supplying nutrients that would otherwise be limited in the surrounding water.
We often view fish as something that increases our nitrate and phosphate, requiring bigger skimmers, refugiums, and other filtration equipment to keep those numbers down. But those same nutrients are also the materials our corals need to grow. The goal shouldn’t be to eliminate them. It should be to find a balance between how much nutrition enters the system, how much gets consumed, and how much we remove.
We’re Already Adding Amino Acids Every Time We Feed
We spend money on bottled amino acid supplements while regularly feeding our fish protein-rich frozen foods. Think about what goes into LRS Reef Frenzy, Rod’s Food, or PE Mysis. These foods contain seafood such as shrimp, fish, clams, and other marine ingredients. And what are these foods rich in? Protein. Proteins are chains of amino acids.
So every time we feed these foods, we’re introducing amino acids into our aquariums, mostly bound together as proteins. Some of that food gets eaten by fish. Some smaller particles may be captured directly by corals. The rest can enter the nutrient cycle as organic material.
When fish digest protein, they break it down into amino acids. Some are used to build their own tissue, while others are metabolized, with nitrogen eventually released as waste. Corals and their symbiotic algae can then use that recycled nitrogen to make their own amino acids and proteins. Microorganisms also break down organic waste, recycling nutrients and releasing organic compounds back into the system.
Interestingly, the 2023 study on fish feces that we’ll discuss next also measured protein content in fish waste. The researchers found considerable differences between species, with some fecal samples containing substantial amounts of protein. That means some protein from fish food can remain in the waste after digestion, potentially becoming available to other organisms as the material is consumed or broken down.
Now, feeding frozen food isn’t chemically identical to dosing free-form amino acids. The amount available to corals, and how quickly it becomes available, will differ. But does it need to be identical? The goal isn’t to reproduce what’s inside a supplement bottle. The goal is to provide the nutrients our corals need. If we’re already introducing proteins, amino acids, nitrogen, phosphorus, and other organic compounds through regular feeding, how much additional supplementation is actually necessary?
What About All the Trace Elements We’re Dosing?
This is probably the most interesting part of the research to me, especially with how popular ICP testing and trace element dosing have become.
ICP test results we get back show us concentrations of iron, manganese, zinc, copper, and other elements, and we then purchase supplements to bring those numbers closer to our targets. But have we considered how much of these elements we’re already introducing through feeding and fish waste?
A 2023 study published in Ecology, Fish Feces Reveal Diverse Nutrient Sources for Coral Reefs, examined feces from 570 individual fish representing 40 species on reefs in Moorea, French Polynesia.
The researchers measured three major organic nutrients:
- Proteins
- Carbohydrates
- Lipids (fats)
They also measured six minerals:
- Calcium (Ca)
- Copper (Cu)
- Iron (Fe)
- Magnesium (Mg)
- Manganese (Mn)
- Zinc (Zn)
These should sound familiar to anyone who’s had an ICP test done. What’s particularly interesting is that the concentrations varied considerably between fish species, even among fish with similar diets.
For example, the researchers found:
Mimic Tang (Acanthurus pyroferus)
This species had particularly high iron concentrations in its feces, averaging 43,015 ppm. That’s approximately 4.3% of the dry fecal material by weight.
Yellowtail Chromis (Chromis xanthura)
This species had particularly high magnesium concentrations, averaging 65,863 ppm in its dried feces.
Sixbar Wrasse (Thalassoma hardwicke)
This species had the highest average manganese concentration reported in the study, at 167 ppm. Its feces also contained relatively high copper concentrations.
Dot-Dash Butterflyfish (Chaetodon pelewensis)
This species had particularly high zinc concentrations, averaging 784 ppm.
Bristletooth Filefish (Amanses scopas)
This species had particularly high calcium concentrations, averaging 398,207 ppm, or nearly 40% of the dried fecal material by weight.
The differences between species are remarkable. That doesn’t mean we should go out and add a school of Mimic Tangs just because our ICP test shows low iron. But it does suggest that their natural diet, and how their bodies process it, may play a significant role in the types and amounts of minerals they release through waste.
The study found roughly an 1,800-fold difference in average iron concentrations between the lowest and highest species measured. Zinc concentrations also varied by approximately 1,800-fold.
Even fish within the same genus could produce waste with significantly different nutrient profiles. The researchers concluded that fish species and feeding groups help determine the nutritional composition of their feces. In other words, different fish recycle different mixtures of nutrients.
Note that an ICP test measures the concentration of elements present in our water at the time of testing. It doesn’t tell us how much of those elements are entering the aquarium through feeding, how quickly they’re being consumed, or how much remains bound in organic matter. So while fish waste clearly contains many of these elements, we still don’t know how much ultimately becomes available to our corals.
Fish Can Directly Contribute to Coral Growth
There’s also scientific evidence that fish can help corals grow.
Researchers studying coral-dwelling damselfish found that corals living with these fish could grow faster than colonies without them.
One study involving Stylophora pistillata observed greater growth in colonies inhabited by damselfish.
Part of the explanation may be the localized supply of nutrients from fish waste.
Fish living among coral branches release nitrogen and phosphorus close to the coral. Instead of depending entirely on nutrients drifting through the surrounding water, the coral receives a more consistent local supply.
There are other benefits too. Some fish protect corals from predators, while their movements can affect water circulation around the colony.
So we can’t attribute all the additional growth to fish waste alone. But the research supports something important: fish aren’t just competing with corals for resources. They can actively contribute to the conditions that allow corals to grow.
The 2023 study on fish feces adds another layer to this relationship. If fish waste contains proteins, fats, and a variety of minerals, then the potential nutritional contribution extends beyond the dissolved nitrogen and phosphorus we’ve traditionally focused on. How much of that material becomes available to corals is still an open question, but it’s another reason to consider fish as part of the reef’s nutritional system rather than simply a source of pollution.
What About Coral Color?
When we said increasing feeding or adding fish often seemed to improve coral coloration, at the time, we were working mostly from experience. But the biology gives us a reasonable explanation for what we were observing.
As covered in my previous article, coral coloration comes from a combination of the symbiotic algae living inside the tissue and the coral’s own pigments, including fluorescent proteins. Those proteins require amino acids, and amino acids require nitrogen. Blue light can stimulate the production of certain fluorescent proteins, but the coral still needs the materials to build them. A coral can receive plenty of light but still struggle to produce tissue and pigments if essential nutrients are limited. Fish help supply those nutrients through feeding and waste.
And as the 2023 research shows, fish feces also contain elements such as iron, manganese, copper, and zinc, which participate in important biological processes. That doesn’t mean adding more fish automatically makes corals more colorful. Genetics, lighting, nutrient balance, and the health of the symbiotic algae all matter.
Too much nutrition can also cause problems. An increase in symbiotic algae can make corals appear brown, even when the coral’s own pigments are still present. The goal is balance. But I think the connection helps explain why many reefkeepers have observed better coloration after increasing feeding or maintaining a healthier fish population.
Putting It All Together
A well-fed reef aquarium receives nitrogen, phosphorus, proteins, fats, and a variety of minerals through food. Fish consume some of that food and recycle nutrients back into the system. Corals may capture food particles directly, while bacteria and other microorganisms break down organic material. Some nutrients are used to build new tissue, some are recycled, and others are removed through filtration.
Compare that with dosing nitrate, phosphate, or trace elements directly into the water. Dosing introduces specific nutrients in measurable amounts, while feeding introduces a much broader mixture of nutrients that become available through digestion, waste production, and biological breakdown. They’re different approaches, and both have their place in reefkeeping.
The 2023 fish feces study adds another layer to this discussion. Researchers identified many of the same elements in fish waste that we’re measuring through ICP testing and supplementing when needed. While that doesn’t tell us how much of each element becomes available to corals, it gives us a better understanding of the nutritional contributions our fish are already making.
Of course, I’m not suggesting everyone should go out and add more fish. There’s a limit to how many fish an aquarium can comfortably support. Overstocking can lead to oxygen depletion, aggression, excessive waste, and poor water quality. The key is maintaining an appropriate fish population, feeding it well, and having the filtration capacity to manage the excess.
I’m also not against supplementation. Not all tank sizes can support that many fish. A tank with very few fish, limited feeding, or aggressive nutrient export may need additional nutrients. Other elements may still require supplementation depending on the system, particularly calcium and alkalinity in tanks with heavy skeletal growth.
No two tanks are alike. There’s no single fish count, feeding schedule, or nutrient number that works for everyone. I hope this article offers a fresh perspective on how fish waste contributes to the nutrition we provide our corals.
My keyword has always been balance. My preference has always been to maintain a healthy fish population, feed adequately, and use filtration to manage the excess rather than trying to keep nutrients near zero and then dosing them back individually.
References and Further Reading
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Van Wert, J.C. et al. (2023). Fish Feces Reveal Diverse Nutrient Sources for Coral Reefs. Ecology, 104(8), e4119. Study of 570 individual fish across 40 species, examining proteins, carbohydrates, fats, and six mineral elements in fish feces.
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The Role of Fish Feces for Nutrient Cycling on Coral Reefs (2023). Oikos. Research on nitrogen and phosphorus recycling through fish feces.
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Allgeier, J.E. et al. (2016). Fishing Down Nutrients on Coral Reefs. Nature Communications. Research examining the relationship between fishing pressure, fish biomass, and nutrient recycling.
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National Geographic: Fish Urine and Nutrient Recycling on Coral Reefs. Accessible discussion of the importance of fish-derived nutrients on coral reefs.
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Effects on Growth and Reproduction of the Coral Stylophora pistillata by Mutualistic Damselfish. Research examining the relationship between resident fish and coral growth.
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Nutrient Supply From Fishes Facilitates Macroalgae and Suppresses Corals in a Caribbean Coral Reef Ecosystem. Research highlighting how fish-derived nutrients can also benefit macroalgae.
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How Corals Build Tissue and Color. ReefnBid Community. Previous discussion of nitrogen, phosphorus, fluorescent proteins, and coral pigmentation.