How To Lower Nitrates In Fish Tank

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Before we discuss how to lower nitrates in fish tanks we first need to understand what nitrates are, how they are formed and signs of high nitrates.

Nitrate (NO3) is a compound based on Nitrogen and Oxygen; it is an inorganic compound that appears in aquariums through a process called nitrification.

This compound is less harmful than ammonia and nitrite, but it can still cause problems in large amounts.

In fish, the main effect of nitrate is that it impairs respiration and makes it hard for your fish to breathe.

How To Lower Nitrates In Fish Tank

The quickest and easiest way to lower nitrates in fish tanks is to do an immediate large water change. This will lower the nitrate spike and give you time to figure out what caused it.

Nitrogen is an essential element for life, but when in high concentration in the environment, like in the water of our aquariums, it can be extremely dangerous for the health of animals and plants, especially for wild fish or fish that are breeding or growing.

We must keep nitrate levels within safe levels.

If it goes outside the ideal parameters, action must be taken quickly to ensure the safety of the tank inhabitants.

In this article, I will cover nitrate in home freshwater aquariums.

How Is Nitrate Formed?

Nitrate (NO3) is the result of the conversion of nitrite by beneficial bacteria in the aquarium, being the final product of biological filtration.

In the same way that the bacteria in the filter convert Ammonia into Nitrite, other types of bacteria convert Nitrite into Nitrate.

You can learn more bout the cycling process here – How to Cycle A Fish Tank

As an aquarium owner, we are responsible for maintaining the physical and chemical balance of the environment that our fish and other aquatic creatures live in.

Part of this includes periodic testing and maintenance.

The most crucial characteristic of nitrates is that they are soluble, so this compound will be found in all your tank’s water.

A nitrate spike can occur when there is a large biological load in the aquarium as a result of inefficient filtration, overfeeding, overstocking or a lack of periodic maintenance, in particular water changes.

Nitrates will almost always be present to some degree in your fish tank. It is only when they reach high levels that things can start to go wrong.

Effects of Nitrate

Effects of Nitrate in My Aquarium

Ammonia is normally the compound that concerns fish keepers the most but nitrates can and should also be monitored.

Nitrate is less harmful than ammonia and nitrite but can still cause problems.

Nitrate poisoning is more a condition than a disease, but it will slowly kill your fish and invertebrates.

Nitrate is a fundamental macro nutrient that supports life. Both aquarium plants and unwanted algae use it for their growth.

Algae blooms can be a visual factor to look out for if the nitrate levels are too high in the tank.

Nitrate accumulates in our aquariums slowly, rising little by little.

When poor maintenance and management practices are not kept up then nitrate can become an invisible killer.

How Do I Measure Nitrate Levels?

A must-have item for all fish keepers is a good water test kit.

Those kits usually contain a nitrate test so that you can monitor this element in your tank water.

High nitrate levels in aquarium water can be a rough indicator of other dissolved nitrogen compounds like ammonia and nitrite.

Effects on Fish and Plants

In fish, the main adverse effect caused by nitrate intoxication is impaired breathing, usually gasping, and the animal lying on the bottom or close to the surface.

When nitrate is found in high concentrations, the fish carries less oxygen in its blood, triggering several secondary problems.

The main symptoms of nitrate poisoning in fish are wheezing and lethargy.

The fish appears unconscious, staying in the same spot, or getting “tired” after swimming short distances.

The most common visible symptoms on the animal’s body are brown spots on the gills and brownish blood.

These are symptoms of moderate to severe poisoning. Lack of appetite also occurs.

Although nitrates are only considered toxic to fish when they are above 90 ppm, toxicity depends on several factors, from environmental factors (temperature, pH, oxygenation, etc.) and even the fish species and age, among other factors.

Nitrate is more toxic to invertebrates than vertebrates.

In aquariums with animals such as shrimp, snails, and suchlike, you need to be more attentive to nitrate levels in their aquariums.            

Nitrate poisoning often causes irreversible chronic problems in animals, especially in fish in the growing phase.

Nitrate accumulation can also cause fish to develop deformities and stunt their growth.

Despite causing severe problems in animals, nitrate is a fundamental compound in the development of plants.

Nitrate is nothing more than nitrogen in its fully available and assailable form to plant species.

Live plants are great at maintaining and reducing nitrate levels in freshwater aquariums.

Nitrate poisoning

Algae, cyanobacteria, and other microorganisms also use nitrate directly.

That’s why, when an algae bloom is happening in one of your tanks, the first thing to do is measure the level of nitrates in the water.

How To Get Rid of High Nitrates In Fish Tank

The safest, quickest, and most efficient way to control nitrate in aquariums is to keep to a constant maintenance routine.

This involves carrying out tests, cleaning the substrate, and performing partial water changes.

That way, you will never have a problem with nitrates or other nitrogenous compounds like ammonia and nitrite.

A water change will always be necessary, no matter the presence of plants or how good your filtration is.

Another factor that can cause high nitrates is overfeeding so make sure you only feed as much food as your fish will eat in 2 minutes. Then remove any uneaten food.

Overstocking your fish tank can also cause high nitrates. You will need to thin out your tank’s population if this is the case. Either re home some fish or take some back to the Pet Store.

Live plants and unique media for nitrate removal or reduction will undoubtedly help, but you should always address the cause directly rather than using this remedy a problem.


Some fish keepers fear water changes and believe the rapid reduction in nitrate can cause their fish to go into shock.

Despite being statements devoid of scientific foundations, this thought is somewhat understandable.

Still, in the circumstance of intoxication by any compound, the rapid reduction of potentially harmful toxins in an aquarium is the only way to correct it.

The best way to avoid severe shocks in the various parameters of an aquarium is to follow a regular maintenance routine and perform partial water changes frequently, constantly conditioning the new water and equalizing temperature, pH, etc., with the water in the tank.

Immediately perform a water change at the slightest sign of a nitrate spike.

Finally, to prevent the fish from suffering any “shock,” carry out large partial exchanges interspersing a few days between each one.

Nitrate levels should start dropping right away!

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Irma Bense is the founder of She has over 42 years of experience in keeping fish.She has kept both freshwater and saltwater fish through the years.She has extensive knowledge in breeding numerous species of fish from Show Guppies, Cory Catfish, German Blue Rams to Apistogrammas.You can learn more about Irma on the 'About' page.

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