Is Ammonia Toxic To Corals? (With 10 Examples)

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We all know that ammonia is a dangerous substance in fish tanks, especially for marine animals such as fish, snails, and shrimp.

But how do corals react to ammonia? Are they sensitive like fish? Or can they actually help with ammonia by consuming it?

These are all questions I have often asked myself in the past. So, to make it easier for you, I decided to collect everything I know about this topic into one article.

Let’s dive right into it.

A picture of a coral illuminated by blue light in a reef aquarium

Is Ammonia Toxic To Corals?

Yes, ammonia is toxic to corals, and high concentrations can result in coral bleaching and even death. However, corals can tolerate higher levels of ammonia than fish, and the exact amount that is toxic to corals varies depending on the species.

In other words, the answer isn’t as straightforward as you think. Ammonia’s relationship with corals is somewhat complicated.

Consider the following:

1. Nitrogen Compounds In Reef Tanks

Ammonia matters to a reef tank because you can’t avoid it. Nitrogenous compounds appear when leftovers, waste, and dead organisms rot.

Fish will also eject ammonia and ammonium from their gills. Healthy tanks have nitrifying bacteria that convert ammonia into nitrite and nitrate.

Common sense will tell you to eliminate the nitrogenous waste completely. However, plants and algae feed on nitrates.

They use the nutrients to grow.[1] Therefore, removing all nitrogenous compounds is not in your best interest.

A visual representation of the process of nitrogen cycling within an aquarium

2. Zooxanthellae Rely On Nitrogenous Compounds

On the surface, the role nitrates play in plant growth seems irrelevant until you realize that corals host zooxanthellae, microscopic organisms that photosynthesize.

This is where the complications arise.

Nitrogenous compounds are a vital source of nutrients for zooxanthellae. However, the state of the nitrogen matters.

Some aquarists think that corals use ammonia, but only because they’ve confused ammonia with ammonium.

Laura Fernandes de Barros Marangoni, Renaud Grover, Cecile Rottier, Christine Ferrier-Pages, and Adalto Bianchini published a paper in Scientific Reports which found that nitrates and ammonium produce opposing side effects in corals.[2]

Corals are sensitive to high temperatures. In fact, rising sea surface temperatures in the wild have bleached many reefs.

According to the study above, nitrate enrichment will prolong coral bleaching in the presence of thermal stress, especially when phosphorous levels are low.

On the other hand, ammonium enrichment can promote photosynthesis, delaying the consequences of thermal stress.

But unlike ammonium, ammonia is dangerous to corals. A paper from Chulalongkorn University in Bangkok, Thailand, proved as much.

The study (C Petchporn, U Bussapakorn, R Sompop) looked at the effects of various temperatures and ammonia levels on corals.[3]

It observed that varying ammonia concentrations resulted in coral bleaching.

Although, it took a combination of the wrong temperatures and ammonia levels to kill the corals in question.

Many aquarists believe that corals can thrive in the face of moderate ammonia levels. Ammonia only becomes a threat when the concentration spikes. This is also true for nitrates.

Coral tissue hosts zooxanthellae that utilize nitrogenous compounds and light to carry out photosynthesis

3. Corals Are Not As Sensitive To Ammonia As Fish

Your approach to cycling will depend on the tank’s inhabitants. If you have fish, cycling is vital.

The practice introduces nitrifying bacteria that turn toxic ammonia into relatively harmless nitrates. Naturally, the process takes between six and seven weeks.

However, a reef tank with only corals doesn’t require cycling. First of all, while corals are living organisms, they differ drastically from fish.

Corals have a low bioload. They will generate CO2 and some waste products, which the algae in their tissue use (for photosynthesis).[4]

However, don’t expect their presence to dramatically increase the tank’s ammonia concentration.

Secondly, corals are not conventional animals. Therefore, you can’t expect them to manifest the same response to ammonia as fish.

Ammonia is dangerous because it destroys hemoglobin, a component of the red blood cells that carries oxygen.

But corals don’t have blood, let alone red blood cells. Therefore, they can survive high ammonia levels.

Admittedly, ammonia becomes a threat to cellular functions in all life forms once you elevate the concentration beyond acceptable levels.

But the ammonia you find in an average reef aquarium is unlikely to harm the corals.

Therefore, cycling is not necessary for a tank that only houses corals. You can still cycle the tank if the water is cloudy.

It is worth noting that high ammonia levels promote algae blooms. In other words, cycling serves a purpose.

An image of Montipora digitata, a coral species known for its hardiness in reef aquariums

How Much Ammonia Is Toxic To Corals?

It is common practice to keep ammonia levels at zero. Admittedly, you can’t remove ammonia entirely in a reef tank.

However, you should reduce it to a level where it becomes undetectable. Quantities that exceed 0.1 ppm are dangerous to aquariums.[5]

But corals are a different matter. You will be hard-pressed to find concrete data showing the exact quantities of ammonia that generally kill corals.

Although the Chulalongkorn University study recorded significant mortality rates among corals exposed to ammonia concentrations of 0.1mgN/L.[6]

But this only tells you how Acropora sp responded. You don’t know whether similar concentrations would harm corals from different species.

Keep this in mind: corals can tolerate higher ammonia levels than fish. The creatures will most likely survive concentrations that hurt and even kill fish.

Therefore, just because your fish are keeling over and dying doesn’t necessarily mean your corals are about to follow.

Naturally, dead fish in a reef tank should concern you. Take it as a sign that your tank requires your immediate attention.

Thoroughly cleaning the substrate through vacuuming is an essential measure in combating ammonia build-up in fish tanks

Pro Tip: To monitor the ammonia, nitrate, and nitrite levels, I personally use the API AMMONIA 130-Test Kit (link to Amazon). It is highly accurate and lasts forever.

Are Certain Types Of Corals More Sensitive To Ammonia Than Others?

Some corals are hardier than others. Types like the bubble coral can survive unconducive environments, which is why aquarists recommend them to beginners.

These hardy corals will tolerate higher ammonia levels for longer periods. In that same vein, some corals are too sensitive to survive minor ammonia spikes.

However, researchers have yet to perform comprehensive studies exploring the different coral types and their response to varying ammonia levels.

Therefore, your best option is to experiment. Expose different coral types to various ammonia concentrations to get a better sense of their responses.

Prioritize coral species with a strong tolerance for the ammonia levels your tank typically attracts.

To make life easier here is a short list of five hardy corals and five sensitive corals, so you can make a wise purchase if you haven’t bought them yet:

Five hardy coral species:

  • Porites lobata
  • Montipora digitata
  • Euphyllia ancora
  • Pocillopora damicornis
  • Stylophora pistillata

Five sensitive coral species:

  • Acropora millepora
  • Seriatopora hystrix
  • Pocillopora acuta
  • Stylophora milka
  • Turbinaria reniformis
On the left is Euphyllia ancora, a coral species known for its resilience, while on the right is Acropora millepora, a comparatively delicate coral

How Do Corals React To High Ammonia Levels?

If your coral is exposed to high ammonia levels, the following outcomes are likely to occur:

  • The coral’s colors will grow dull.
  • The polyps won’t extend.
  • Anecdotes from some aquarists mention corals that remained closed indefinitely.
  • The end result of exposure to high ammonia levels for extended periods is coral bleaching.
  • The corals will eventually die.

Does Ammonia Have Long-Term Effects On Corals?

Like fish, corals that manifest worrisome signs such as dull colors because of high ammonia levels will recover once conditions improve.

Don’t forget that corals require the correct temperature, salinity, hardness, and phosphate levels to thrive.

If your corals continue to deteriorate despite all the water changes you’ve performed and conditioners you’ve added, there is something else.

Consider the possibility that your reef tank has the wrong parameters and that high ammonia levels are not responsible for your coral’s poor health.

Do Corals Consume Ammonia?

As I mentioned earlier, corals and zooxanthellae live in symbiosis.

Zooxanthellae use nitrogen compounds to produce nutrients for the corals, and in return, the corals provide the zooxanthella with a safe environment.

But corals and zooxanthellae don’t use ammonia (NH3), they consume ammonium (NH4+).

While ammonia is toxic to corals and other living creatures, ammonium is not.

An excellent video discussing ammonia, nitrate, and nitrifying bacteria in reef tanks

How Do I Get Rid Of Ammonia In My Reef Tank?

If your reef tank features high ammonia levels, these steps will help you out:

  • Perform regular water changes, changing 10-15% of the water weekly for smaller tanks and up to 20% for larger tanks, matching the new water’s parameters to the tank’s conditions.
  • Vacuum the gravel during water changes to remove debris.
  • Manage the aquarium filter, including replacing old filter media, cleaning the filter, and ensuring the filter is not too powerful or weak.
  • Use ammonia removers, such as ammonia chips or filters with ammonia removal systems, to provide a long-term solution to the ammonia issue.
  • Use caution with large water changes, as they can affect the bacterial community structure and should only be used as a last resort during emergencies.

Pro Tip: Combat ammonia in your coral tank with my complete guide on aquarium ammonia, covering causes and solutions. Keep your coral healthy!


If you’re in a hurry, here’s a quick rundown of the key points mentioned above:

  • Ammonia is toxic to corals, but its relationship with corals is complicated due to its role as a nutrient for zooxanthellae, microscopic organisms that photosynthesize within coral tissue.
  • Corals can tolerate higher levels of ammonia than fish, but ammonia concentration must be reduced to a level where it becomes undetectable.
  • Nitrogen compounds are common in reef tanks, and some corals rely on them for nutrition, but high levels of ammonia can cause coral bleaching and death.
  • Corals do not have blood and red blood cells; thus, ammonia does not affect them in the same way as it does fish.
  • Some types of corals are hardier and can survive unconducive environments, while others are too sensitive to survive minor ammonia spikes, but research on their response to varying ammonia levels is limited.