Nitrate Poisoning in Aquarium Fish – Possible Causes and Remedies

There comes a time when you may observe your fishes exhibiting weird behavior.

They may stay still, appear disoriented, swim in circles, lay at the bottom of the tank, and have no appetite.

They may gradually start bending, curling, and have a crooked spine. All these are signs of nitrate poisoning and needs immediate correction.

Nitrate poisoning occurs due to the build-up of nitrates gradually, over time, in the aquarium.

What causes Nitrate Poisoning in an Aquarium Fish

As a result of eating, fishes constantly produce waste.

Another source of waste build up in the aquarium water are the uneaten food particles, the dead plant leaves, and other organic matter.

All this waste decomposes into ammonia. Nitrosomonas, which are a colony of bacteria, feed on ammonia. Nitrites are released as a byproduct.

There are other bacterias ready to feed on Nitrites.

They are Nitrobacter that convert nitrites into nitrates. Nitrates remain in the aquarium until they are removed. These nitrates can accumulate and overtime cause nitrate poisoning.

Nitrate poisoning occurs slowly over time when the fishes are exposed gradually to the rising nitrate levels. If these nitrates are not removed, the fish can even die.

When the nitrate content is more than 20 parts per million in water, the fish will start showing signs of nitrate poisoning.

Some of the common causes of nitrate poisoning are:

Overfeeding the Fish

Overfeeding is the most common cause of nitrate poisoning in the aquarium.

Giving the fish more than it requires leads to uneaten food getting decomposed in water, and an increase in waste production.

As the food gets decomposed by aerobic bacteria, nitrates are released as a byproduct. Nitrates get accumulated, leading to their buildup.

Also read: How to Feed Aquarium Fish When On Vacation?

A fish tank that is overstocked

Having too many fish in the aquarium is another common reason for the nitrate levels to become high.

A lot of fishes and plants in the aquarium release a high amount of organic waste.

This waste remains in a closed system space converting into nitrates that remain in water leading to nitrate poisoning.

Delay in cleaning and maintenance

Most of the time, the buildup of nitrate in the aquarium is due to a delay in cleaning and maintenance.

Slowly, the build-up of nitrates gets too high for the small spaced aquarium.

The aquarium needs to be cleaned properly and timely to ensure the effective removal of nitrates. Even the gravel needs a vacuum to remove all the waste.

Delay in water change

Fishes need freshwater in order to be healthy. The timely change of water ensures the removal of nitrates and good mineral and oxygen supply to the fishes.

The waste needs to be diluted on time to prevent nitrate build-up.

30 to 40% of the stale water needs to be replaced from time to time to make sure that the waste is effectively being removed from the tank.

Using nitrate-rich tap water for aquarium

Despite the state government filtering tap water, nitrogen ions may remain unfiltered and leach into tap water.

With the aquarium water being confined to a small space, leaching small amounts of nitrogen in tap water can lead to the accumulation of large amounts of nitrates in the aquarium.

Clogged filter media

Sometimes the reason for high nitrate build-up can be unclean and clogged filter.

A choked media will not work very well in filtering the water. This will lead to an increase in nitrogen ion content in the fish tank water.

Absence of live plants in the aquarium

Plants keep the fish healthy by improving water quality and passively using the excess nitrates in the water.

They use nitrates as a source of food for growth. However, care should be taken to prune the plants.

Their number should be according to the aquarium size. Excess or few plants can be a cause of organic waste, leading to high nitrates.

Symptoms of Nitrate poisoning in Aquarium Fish

Since nitrate poisoning is a gradual process, it is not uncommon for only a few fishes to exhibit one of the symptoms.

As the levels increase, all fishes will start showing symptoms. If the nitrates are not removed on time, it causes the death of the fish. This can happen in a few days or weeks.

Very high nitrate levels lead to the death of the fish within 24 hours of maximum exposure.

Nitrates come in contact with the fish through the gills rather than the skin. This allows them to enter directly into the bloodstream.

They oxidize the iron atoms in hemoglobin and reduce oxygen supply to the bloodstream and tissue. This condition is called methemoglobinemia.

Some signs of nitrate poisoning are :

  • Loss of appetite
  • Staying still
  • Dazed
  • Disorientation ( swimming in circles, uncontrolled swimming- swimming on side or upside down)
  • High respiratory rate accompanied by rapid gill movement
  • Laying down on the bottom of the tank
  • Fading color and a pale look
  • Swim bladder disease

In advanced stages, a curl with a crooked spine or bent positioning can be seen in the whole body from head to tail.

Some complications that can occur from severe nitrate poisoning are dysfunctional reproductive organs (lower chance of reproduction), weakened immune system, and damage to the nervous system.

The severity of the symptoms depends on the type of fish and the intensity of exposure.

Cures for nitrate poisoning in aquarium fish

The fish needs to be treated on an emergency basis. Severe poisoning can cause death to the fish within 24 hours.

Test the level of nitrate content in water with the help of test strips.

The critical thing to remember is that the fish is already stressed.

The gradual removal of nitrates is necessary to avoid further stress to the fish. This helps to give them time to regulate body fluids (osmoregulation).

Perform a gradual and controlled water change

Start a controlled water change to get quickly rid of nitrate contamination.

A 5% water change should happen per hour. Do this until you get the desired effect in the water nitrogen test strip.

Commercial Nitrate remover

In case of emergency, when the nitrate level can kill your fish, buy a commercial nitrate remover.

It is the quickest way to reduce nitrate levels in a short period effectively. They do this without causing any stress to the fish.

This method is efficient in removing high levels of nitrate content in water.

Reduce the temperature of the tank

Until oxygen levels are raised, and nitrate levels are reduced, reduce the water temperature of the fish tank.

You can do this with the help of an air conditioner in the room or use a fan aimed at the tank.

Alternatively, you can place frozen bottles in front of the tank.

Increase water depth and add more filter pumps

Increasing the depth increases the water pressure. Increasing water pressure helps in removing the excess nitrates.

High nitrate levels reduce the oxygen levels in the water. Increase the oxygen level by adding more filter pumps.

They will allow water to absorb more oxygen.

Add floating plants

Add floating plants to the aquarium. Nitrate is a plant food used to make energy for their growth.

Introduce nitrate-absorbing plants to the fish tank (such as giant duckweed, hornwort, water sprite, etc.)

Floating plants get more atmospheric carbon dioxide. The more carbon dioxide a plant can get, the better it will use up the nitrates in the aquarium.

It is, therefore essential to place floating aquarium plants in the tank.

Change tap water with R/O water while filling the Aquarium

Tap water is sometimes the main culprit giving a high concentration of nitrates.

RO water undergoes treatment and has most of the impurities removed, including a higher concentration of nitrates.

It uses the technique of reverse osmosis, making the tank free from all impurities.

Either buy the RO water for your fish tank or invest in RO/DI water filter.

Also read: Can Distilled Water Be Used in a Saltwater Aquarium?

Use a refugium

Refugium is another tank that connects with the main tank with the help of tubes and a pump. The tubes will circulate water through both the tanks.

Use a lava rock inside the refugium. Being porous, it will help in the growth of denitrifying bacteria in its pockets.

They will convert nitrate into nitrogen molecules. Nitrogen molecules are harmless to the fish. Try Growing algae inside the refugium.

You can do this with the help of LED lights. Algae will suck up the excess nitrates from your tank.

Feed alternative day

Make your fish fast one day and give it food every alternative day.

Feeding this way will do no harm to any fish. In fact, it will ensure there is no overfeeding and buildup of waste content leading to Nitrate poisoning.

Do not overstock

Do not overstock your tank with fish and plants.

Maintain a balance between the size of the tank and the number of live plants and fishes. This way, the water will not build up high nitrate levels very fast.

Make sure the ppm of nitrates remains apt until your next water change.

Adding fishes and plants, disregarding the size of the aquarium, and the volume of water will lead to faster organic waste build-ups.

It becomes difficult to change the water at such frequent intervals.

Use a vacuum for deep cleaning

Vacuum the substrate present in the aquarium every other day until they are all clean.

Do this in intervals and only about 30-35% so that beneficial nitrifiers get time to re-establish themselves.


Curing the fish may take a few days or a few weeks according to the severity of the nitrate poisoning.

If high nitrate levels are a frequent problem in your tank, keep checking ppm levels frequently. Keep live plants as a preventive measure.

Follow the principle of feeding every alternative day. Remember not to overstock your aquarium.

Regularly clean and maintain the tank. Clean the filter and the substrate media.

Change water content weekly or biweekly. Follow the above precautions with care. This way, you will be able to care for your fish in the best possible manner. All the best!

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