My new discus died… Because of nitrates in my aquarium?


Nitrates in the water of my discus aquarium? Is it dangerous?

Like other fish, we know that discus can adapt to different water parameters.

Sometimes I also met some cases where maintenance conditions, management of water quality, challenge the known requirements of this fish.

However, we must be careful not to consider these exceptional cases as examples to follow. Because these very often remain isolated cases; whose "success" usually lasts only a time.

Here's an example that I have met several times: The one of the nitrates…

I draw your attention to the fact that this article presents products that are not part of any sponsorship or monetization for their presence in this article.

What nitrates level for discus?

As you discovered it in the article " Understand the key water parameters of an aquarium" ; Nitrates are the result of the biological filtration.

They accumulate naturally in water and are tolerated by fish.

Nevertheless, you will have to monitor the value of nitrates…

Keeping in mind that the content of nitrates increases rapidly when there are a lots of discus in the aquarium…

Red Melon discus 2015 Yann Hoiret

To provide good quality water (Regarding the Nitrates parameter); I advise you to avoid exceeding 25mg/L.

Discus can smoothly evolve to higher levels, but this causes decline in immunity (Disease resistance), or retards growth of young fish.

Furthermore, we must not forget that in case of sharp drop of the oxygen content; Nitrates can turn into Nitrite (toxic).

This parameter should be monitored regularly. Generally, a test performed once a month remains sufficient. A test that could also be associated with the measurement of phosphate, which tends to cause the same problems.

Discus poisonous by nitrates?

Sometimes discus lovers provide a maintenance that becomes over time hazardous…

Confident of their success, it happens that the rhythm of their water changes become irregular; and they do not realize that environmental quality deteriorates gradually.

Often; because of lack of knowledge, by habit, or even by negligence the simple water test is no more done.

But aquarium water parameters are not fixed; more or less, they can change quickly over time.

Then comes the day when these hobbyists wish to expand their group of discus. And here… it's a catastrophe. They are surprised to see their latest acquisitions not being able to adapt in the aquarium and die very quickly…

They do not understand that one (or more) discus purchased recently can die so quickly while others living in the aquarium for a long time appear healthy…

Is it the fault of the seller?

You sold me diseased fish?
You sold me sick fish!!!

If this happens to you: After dismissing the possible causes related to transport and acclimatization problems, I invite you to simply test the water of your tank ...

We often find some explanations there!

In the case of nitrates; they accumulate over time in the aquarium. To avoid high values, you have to ensure regular and fairly substantial water changes.

If these water changes are not properly done, the nitrates value of the Aquarium tends to increase slowly but surely over the months.

The fish in the aquarium support this change because it is progressive. The change of their environment is not brutal; they adapt…

It happened to me to observe discus having a vigorous behavior with a good appetite in a tank with over 100mg/L of nitrates.

However, this situation can not last and it is ABSOLUTELY NOT RECOMMENDED to acclimate new discus in this type of water even if those already in are doing well !

Because with stress of transport , acclimatization, Competition to fit within a new group adds a strong environmental stress. We then minimize the chances of adaptation!

Of course, many other parameters change and can cause problems of acclimatization to the new fish.

But the example of death due to high value of nitrates is not uncommon… While it is yet easy to avoid!

In this case, the seller is not to blame…

How to remove nitrates in my aquarium?

You understood, nitrates (Beginners often confuse with "nitrites") are not of comparable toxicity to ammonia or nitrites.

Also note that all species of fish do not have the same degree of sensitivity regarding the amount of nitrates in the aquarium.

Nevertheless, and for the reasons you have seen above, we should worrying about keeping low values. For this, there are several methods…

Water changes to reduce nitrate in my aquarium

water storage example for several aquariums discus

And Yes… We can not emphasize this enough, but discus fish like water changes!

These allow, among other to stabilize the amount of nitrates. Water containing nitrates is removed and replace with water that does not contain any.

Thanks to the monitoring performed using a nitrates test; the aquarist can define if the frequency and proportions of water changes are sufficient.

As part of an ornamental maintenance, it is expected to change at least 30% of the aquarium volume per week.

When the breeding aquarium having high fish densities require the same volume (or even more) every three days.

You will understand it, it will be directly linked to the amount of fish hosted and more particularly to organic pollution generated by all the "aquarium system".

It is also clear that the impact of the pollution caused by eight discus maintained in an aquarium of 250 l will not be the same if they are hosted in 600 liters of water.

This is why I insist on the need of proper monitoring water parameters in the aquarium.

However, pay attention about two important things…

Of course you will have to be sure that the water used for the aquarium partial renewal is itself free of nitrates.

Indeed, tap water is considered safe until a value of 50 mg/L in France. This is of course above to what is recommended for our discus…

Caution therefore thoroughly check the water you use… All regions and water networks are not equal. Not to mention that there may be seasonal fluctuations.

Monitoring of drinking water quality in France

Website of the ministry responsible for monitoring the quality of drinking water in France

It is not a reason to be alarmed but to be aware of certain peculiarities that may exist.

To my French friends readers, I let you check the water quality of your city on the website of the Ministry of Solidarity and Health.

For my abroad friends readers, please tell me in the comment section (Below) if there is such websites in your country.

Do not confuse addition of water and water change…

Why change the tank water
Simplified diagram of the impact of additional water due to evaporation

The second thing I wanted to mention concerns the clear definition of water change in the aquarium.

Very often I hear aquarists saying: " The water in my aquarium is regularly changed; as soon as it evaporates, I refill it!"

And this is also a classic mistake. Because it must be borne in mind that what evaporates is only the water H2O.

Everything dissolved in water does not evaporate (minerals, nitrates, phosphates etc…). On the contrary, these elements remain and concentrate. In other words, water supplement linked to evaporation can not substitute to a "Water change".

Changing water of the aquarium means replace in equal volume. A replacement that must also be performed with the help of a sand vacuum for tanks having sand at the bottom.

We take out water "H2O" which contains (Nitrates, Phosphates, minerals etc.…) to replace it with water containing minerals (With a to be defined quantity), without nitrates or phosphates.

To complete evaporation, use for example pure RO water (Without or very few minerals).

How to lower nitrate levels in an aquarium
The water change allow to replace water loaded with undesirable substances and stabilize some parameters (For example the mineral content).

Filtration to reduce nitrate in the aquarium

Exept from the essential action of the water change, there are some ways to low the nitrates level in an aquarium, or even maintain this content at the lowest.

One of these methods consist of modify or "customize" the filtration…

The filtration bacterias to reduce the level of NO3- ?

Siporax WILL bacterial support to remove nitrates in the aquarium
The Sera siporax bacterial media

Indeed, there are brands offering filtration elements or bottled bacterias ensuring a reduction of nitrates.

Regarding the filtration elements, the natural concept taking place there is called denitrification.

Amongst those filtration media we can mention the " De Nitrates " from Seachem or the " Siporax " from Sera Gmbh well known in Europe.

Close up sera siporax bacterial Support aquarium filter

The siporax is in my opinion not really well known for its ability to remove nitrates from water…

There are even studies demonstrating the efficacy of Siporax on decreasing the nitrate level.

Here are below videos provided by the manufacturer. And presented as you will see by Dieter Untergasser a well-known by Discus experts… (French or English)

SIPORAX French Version (With a beautiful German accent!)
Siporax English Version

Denitrification thanks to a polymer…

Balance Balls Proline Tetra Anti nitrates pour aquarium
Balance Balls from Tetra company

Another method proposed for example by the companies Tetra and JBL (JBL BioNitratEx) relies on the addition of a polymer in the filtration.

Being in form of balls to add in the filtration, This material becomes over the next few days a substrate for a thick biofilm. Under this one, the oxygen content become very low, making denitrification possible with the action of specific bacterias.

Having tested the « Balance Balls » from Tetra right after he was available few years ago, its effect in a professional situation (high fish density) was notable.

Below is a little video made by Tetra showing the process .

Nitrates removing resins

Various brands offer their removing resins absorbing nitrates ions you can regenerate with salt.

Among them we can mention for example the "JBL NitratEx".

Resin anti nitrates

This method should in my point of view be used as a rapid response to a situation became urgent to settle.

A tool that can be considered interesting for a time to regain proper management of the aquarium water. See also: Which water for discus

Plants to remove nitrates from the aquarium?

Heckel Discus in planted aquarium at Eddy Leysen Belgium
Wild Discus kept in a planted aquarium (At Eddy Leysen place in Belgium)

Aquatic plants

Aquatic plants will soak up the nitrates in the stock that has accumulated in the aquarium. These have indeed nitrogen needs for their development.

But unless you have a luxuriant vegetation (This is rarely the case for an discus aquarium); we can hardly count on them for a significant purification.

Moreover, According to Diana Walstad (USA) they prefer even other nitrogen sources such as ammonium.

Diana Walstad presents in his book " Ecology of the planted aquarium "A phenomenon less known that the majority of aquatic plants prefer to find nitrogen (N) in ammonium or nitrite rather than nitrates…

The emergent plant of the type Pothos

Pothos to remove aquarium water of nitrates

If you browse the Internet, you will find lots of videos and articles referring to the ability of "Pothos" (Epipremnum aureum) to absorb nitrates in water.

A study available in the "National University of Singapore" of 2010 indeed demonstrates that this plant is able to significantly reduce the nitrates content in water .

On the other hand, These good results have to be analyzed to assess the impact that this approach might have on an aquarium.

In this study, the decline of nitrate value (phytoremediation method) is obtained through the action of eight cuttings of two leaves immersed in four liters of water. This studied on various concentrations going up to 200 mg/L nitrates.

Study University of Singapore on Pothos
Stydy from LIM MEI HUI, Daphne 2010

It will take eight weeks to obtain a value of 40 mg/L of nitrates starting from an original concentration of 200 mg/L. While it will take only four weeks to eliminate up to 97% from an original value of 50 mg/L nitrates.

The question of the interest to use Pothos's with the main objective to lower the level of nitrates value in a discus aquarium can be discussed.

If we extrapolate for exemple the number of plants required to purify the nitrates in a aquarium of 350 litres (92 gallons), this would represent a huge plantation facility…

Without also forgetting that the study is performed on a fixed starter nitrate amount while the nitrate value in an aquarium don't stop to grow.

In addition, many factors such as light (intensity, period, quality), temperature etc.… will be elements that greatly influence the ability of the plant to capture nitrates from the water.

I think we have to remember here that it has been scientifically proven that the "Pothos" (Epipremnum aureum) captures nitrates, but let us keep a sense of proportion about the effect for a use on large volumes such as those used for the discus maintenance.

Pothos can however represent a good complement of purification (In addition to offer other advantages not discussed here); and their decorative aspect is indisputable.

To summarize about Nitrates in the Aquarium…

The nitrates in the aquarium

You will understand it, this water parameter will remain an element to monitor regularly throughout the life of an aquarium.

Their toxicity has no comparison with other organic compounds such as ammonia or nitrites. But like other elements such as phosphates, those will weaken the fish.

A high nitrate content can however cause rapid death of a newly purchased discus.

The presence of nitrates in the aquarium should be controlled mainly through natural techniques such as regular water changes of frequency and amount adapted to each case. But also with the use of filtration materials allowing the denitrification process; a natural phenomenon obtained by the bacterial activity.

In high doses, Nitrates can "reconvert" into very toxic nitrites.

Their presence is needed in low doses for the general balance of the planted aquarium.

Some plants can also help the aquarist to complete the methods mentioned above.

Share your experiences!

The article comes to an end…

If you have reliable methods to reduce nitrates content in the water of your aquarium; feel free to share them in the comment section of this article.

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About Author

Discus Breeder and Discus columnist, Yann Hoiret share on the website Fanatik-Discus his experience and passion for the King of aquariums. From 2014 He worked as a writer for the french magazine "AquaMag", the German magazines "Discus Live" and "Discus yearbook". He participated as an international judge in France Discus show 2014 and 2016 in Arvert, au Greek Discus Show 2014 in Athens, Discus Show in Paris Grigny 2015, 2017 and 2019, au FIDO concurso 2016, 2017 and 2018 Spain, au Nordic Discus Show 2018 Sweden, au France Discus Show 2018 Cognac, Discusvrienden the discus competition 2018, 2019, 2022 in the Netherlands.


  1. Mickaël on

    Hi Yann, great story thank you again

    Well containment


  2. Yann Hoiret on

    Good evening and thank you again for your comments!
    It's always a pleasure to have your feedback… 😉

  3. Yann Hoiret on

    Hello Gary! Thanks for your comment!
    I guess sometimes a simple diagram helps to better understand…
    Really appreciated, thank you 🙂

  4. Yann Hoiret on

    Good evening Martial,
    Thank you for your comment, it's always a pleasure to know that an article is useful to Fanatik-Discus readers!

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