Tips for dealing with high nitrate levels in the aquarium

Do you know how to set nitrate levels in your fish tank and why should you bother? Although most fish keepers understand the deadly effects of ammonia and nitrites, safe levels of nitrate in aquarium water are often overlooked. However, if nitrate levels are allowed to get too high, your fish and other tank residents will suffer.

High nitrate levels can be a problem in an aquarium when regular water changes and other maintenance are neglected. So do you know what nitrate level is too high? And can you recognize the signs of nitrate poisoning in thefreshwater fish?

In this guide, we answer those crucial questions and show you how to reduce nitrates in your aquarium.

Where Does Nitrate Come From?

Nitrate is produced as a byproduct at the end of the nitrogen cycle, and no matter how diligently you maintain your fish tank, there is always some amount of nitrate present in the water. Did you know that clean tap water contains nitrate? Well, yes, and the amount varies depending on where you live.

I couldn’t figure out why my aquariums were still showing nitrate levels of 20ppm (parts per million) right after partial water changes and heavy vacuuming. So, I tested my tap water, only to find out that it contains nitrates! Who knows?

In the US, tap water can contain nitrate levels as high as 40 ppm, so if your ideal baseline level is 10 ppm, you may need to consider other sources of nitrate-free water for your tank. Nitrates are also contained in rainwater, which is why rainwater is such a good idea for plant growers, but not such a good idea for your aquarium.

In your fish tank, nitrates are produced from decaying plant material, accumulated fish waste, decaying fish food, and general detritus. Also, if the filter media is not properly maintained, the bacteria it contains cannot cope with the load, so nitrate levels rise.

Safe Nitrate Levels In The Aquarium

In the natural environment, nitrate levels in water are generally very low, typically below 5 ppm. However, in the closed habitat of a fish tank, nitrate levels can quickly rise to levels that are dangerous to your fish.

Saltwater Tank Nitrate Levels

In saltwater fish tanks, nitrates are very detrimental to invertebrates and corals and therefore need to be kept at very low levels, ideally below 5 ppm. However, marine fish can tolerate higher nitrate levels of up to 40ppm, giving you a bit more leeway if you have a fish-only setup.

Problems can occur in marine installations, as frequent water changes designed to reduce nitrate levels mean more salt must be added to the tank. Consequently, some fish keepers only fill their tank with fresh water to replace what is lost through evaporation.That doesn’t reduce the nitrate in the tank water, allowing nitrates to gradually build up to problem levels.

Freshwater Tank Nitrate Levels

In a freshwater aquarium, the nitrate level in the water should be below 25 ppm and certainly not above 50 ppm. If you are raising fingerlings or trying to control algae growth, the nitrate level should ideally be below 10 ppm.

How is the Nitrate Test Performed?

Monthly testing of your aquarium water for nitrate concentration is absolutely essential!

In new tanks, ammonia, nitrite, and nitrate levels can rise rapidly if the tank has not been fully cycled, and levels should be checked regularly to monitor whether the cycle has been completed successfully. If you have an established tank, testing the water is a crucial part of your routine aquarium maintenance regimen, and you should always have a testing kit on hand.

If you can’t afford a test kit, ask your local fish store how much they charge for water tests. Some larger stores will test your tank water for free, or may charge a nominal monthly fee which is often cheaper in the long run than buying test kits.

The testing process

All nitrate test kits work in a similar way.

Test kits generally come in two forms; strips of liquid or paper. You can buy master test kits that include tests for water pH and hardness levels, as well as for ammonia, nitrite, and nitrate, though some of the tests are often not used and it may be cheaper to buy individual test kits. for specific things, like nitrates.

paper strip tests

Paper strip tests are easy to use and inexpensive. Take a sample of water from the tank and dip one of the paper test strips into it. Wait a few minutes, and then compare the color of the strip to the color chart provided with the test kit. The color of the strip tells you roughly how much nitrate is in the water sample you used.

liquid tests

Liquid test kits are generally preferred over paper strips as they are found by hobbyists to be more accurate, as long as the manufacturer’s instructions are followed. The test kit comes with a small test tube, a pipette, a vial of liquid test medium, and a color chart. Put some tank water into the test tube, add a few drops of test fluid, shake the tube to combine the contents, and then wait a few minutes for the test to work. The water in the tube changes color, indicating the concentration of nitrate in the water sample.

I always test my tap water too, so I can see how much nitrate it contains, compared to the nitrate levels in my tank water. So if my tap water contains 20 ppm nitrate and my tank water has 25 ppm nitrate, then I know the tank is pretty clean. If the tank water has 40 ppm nitrate, then I know I have work to do!

The Potential Dangers of Excessive Nitrate Levels

Excessive nitrate levels can have detrimental effects on life in your aquarium.


In freshwater and marine tanks, your fish will start to be negatively affected when nitrate levels reach 100ppm, especially if you don’t take steps to correct that. Fish are stressed, which makes them more susceptible to disease and parasite attack and interferes with reproduction. Young fish and fry will not grow properly and many will die.

Additionally, high nitrate levels are also associated with poorly oxygenated water, which will cause more stress in the fish.

When choosing fish to add to your tank, always check the nitrate values ​​for the species, as tolerance levels vary. Test your tank water to make sure any new fish are happy with the water parameters, and ask the fish store to test your water as well so you can see that the numbers are close before you make your purchase. That way, the chances of the fish getting nitrate shock or nitrate poisoning are greatly reduced.

algae and plants

If you’re struggling to control algae in your aquarium, that could be due to high nitrate levels, as the two are related. Nitrate levels as low as 10ppm will stimulate algae growth, and the algal blooms you see in new setups are often caused by poor tank cycling and consequently high nitrate concentration.

Plants use nitrate as an important source of nutrition; however, if the levels in the water rise too quickly, your plants will become covered in algae and eventually suffocate and die.

Nitrate Poisoning vs. Nitrate Shock

Excessive amounts of nitrates in the aquarium can result in catastrophic damage to your livestock. There are two forms of nitrate toxicity:

Nitrate poisoning

Nitrate poisoning is a chronic problem that occurs when nitrate levels gradually increase over a period of weeks. The problem is usually due to poor tank maintenance, overstocking, overfeeding, or a combination of all three.

The effect of nitrate poisoning is generally fish death, and juvenile and marine fish are affected by comparatively low levels of nitrates in the water. In cases of nitrate poisoning, often only one or two fish are initially affected. If you don’t test the water, the cause of fish sickness can be missed, until more fish succumb, and eventually fish kills begin to occur.

Depending on the species of fish, death may not occur for a few days to a few weeks.

nitrate shock

Nitrate shock is the term used to describe the sudden exposure of fish to a high concentration of nitrates. However, the same condition can occur if fish are exposed to a sudden and dramatic drop in nitrate levels.

Nitrate shock usually causes fish to die within 24 hours, and more often than not, owners are not aware of the problem until it is too late. Nitrate shock usually occurs when new fish are introduced to a tank that already has extremely high levels of nitrate. The fish are shocked by the poor quality of the water, even though the existing residents are not affected, as they have gradually become conditioned to their environment over time.

You can also cause nitrate shock by doing major water changes in an established tank that has very high nitrate levels, as the sudden drop in nitrate levels will shock your fish.

Symptoms of Nitrate Poisoning in Fish

The following symptoms may indicate a nitrate problem in your aquarium:

  • Lethargy, including lying on the substrate.
  • Lack of appetite and rejection of food.
  • Rapid respiratory rate and movement of the gills.
  • Disorientation and inability to swim properly.

In advanced cases of nitrate poisoning, fish may curl up from head to tail.

If you notice any of these symptoms in your fish, test the water immediately to make sure the nitrate levels are within acceptable parameters for the species of fish you have in your aquarium.

If nitrate levels are too high, there are several things you can do to alleviate the problem and save your fish.

Nitrate Poisoning Treatment

Your first action should be to reduce the nitrate levels in the tank. However, water changes should be controlled and gradual so you don’t shock your fish as described above. During the first 24 hours of treatment, do not feed the fish. Most likely, the fish will not eat the food anyway, and the uneaten food will only contaminate the water, making the problem worse.

Test the tank water for nitrates. That will give you a baseline figure so you can see how many water changes may be necessary.

Your target number is below 20 ppm. However, the reduction of nitrates in the water must be done gradually, removing only up to 5% of the water every two hours. Continue changing that amount of water until you have replaced about half of the total water in the tank. That will drastically reduce nitrate levels, but at a rate that won’t surprise the fish.

When you have finished the first round of water changes, retest to see how much the nitrate level has been reduced. If you still have too high levels, you will need to repeat the process the next day.

contributing factors

There are several factors that contribute to high nitrate levels in the aquarium, and understanding them is the key to preventing future problems.


Overfeeding your fish is not good for their health. Some species, such as bettas and fancy goldfish, can suffer from conditions such as bloating and constipation if given too much food. The more food the fish is given, the more nitrate their digestive processes produce, which increases the toxin load in the water and puts a strain on their biological filtration system.

Additionally, any uneaten food will disappear into the substrate where it will gradually break down, contaminating the water and adding to the cocktail of toxins that are present in the water.

Make sure you feed your fish the correct diet and offer enough food to last the fish for a minute or two. Feed once or twice a day and include one fasting day a week for the fish’s digestive system to rest.


Never overload your fish tank. The more fish you have in the tank, the more waste they will produce and the more nitrates you will have in the water.

As a general rule of thumb, you should allow a gallon of water for every inch of fish, remembering to leave extra room for juvenile fish to grow. Before buying new stock for your tank, check the maximum size the fish will reach when fully mature. Also, keep in mind that some species are territorial and need more space than others. A crowded tank creates stress, which can be exacerbated by poor water quality.

dirty filters

If you allow the mechanical filters to become dirty and clogged with debris, the impeller will not function properly. That can cause poor water circulation around the tank and pockets of dirty water and high nitrate levels can develop.

Clean filters at least once a month and replace carbon filter media regularly to help keep nitrate levels in check.

decomposing plant material

Keeping live plants in your aquarium is a great way to control nitrate levels, as live plants use nitrates as fertilizer for their growth. However, you must maintain your plants and keep them tidy. Remove dead leaves and reduce excessive plant growth to ensure that dead plant material does not collect at the bottom of the tank, where it will decompose, increasing the nitrate level in the water.

How to Reduce Nitrate

There are many ways you can reduce and control nitrate levels in your aquarium:

keep plants alive

Keeping live plants is a very good long term strategy to keep the nitrate levels in your tank balanced. Live plants extract nitrate from the water and use it as a fertilizer, which helps reduce the amount of nitrate in the environment.

Keep your tank clean

Keeping your aquarium clean and in good condition is the best way to control nitrate levels. Fish waste eventually produces nitrate, so be sure to vacuum the substrate thoroughly every week to remove debris and food debris that will otherwise break down and contaminate the water.

Use filter media that remove nitrates

If your tank has a persistent nitrate problem, special nitrate removal filter media can be an extremely helpful tool. These products are available online or at good aquarium stores.

Perform weekly water changes

You should do partial water changes every week to keep the nitrate levels in the tank under control. However, if your tap or well water has high nitrate levels, you may want to use DI (deionized) water or RO (reverse osmosis) water for water changes. However, both options have no mineral content, so the pH and hardness of the water may change, and you may need to add a pH buffer or mineral supplement to correct parameters to suit your fish.

Alternatively, mix DI or RO water with tap water to create a mix that matches your desired water parameters.

Use a denitrating filter

Special nitrogen-removing filters called denitrators can be used if you have a problem with high nitrate levels in your tank, but these can be very expensive, and a nitrate-reducing filter media is a better option, although you will need to replace it periodically.

Install a shelter

A refuge is basically a remote location that is home to a population of wildlife. When used as part of a nitrate control strategy for marine and reef tanks, a refuge provides a separate environment in which macroalgae can grow.

Macroalgae are extremely effective at removing nitrate and phosphate from the water as it passes through the shelter, reducing levels that would be harmful to your livestock. Just periodically remove the algae or seaweed harvest and the nitrates with it.

use microbes

Various types of live microbes can also be used to monitor nitrate levels. Microbes use nitrate for biomass or convert it to nitrogen gas which harmlessly evaporates. There are two forms of microbes; aerobic and anaerobic.

Aerobic microbes are made up primarily of heterotrophic bacteria that absorb nitrates quickly but need to be «fed» some form of carbon, such as ethanol. Unfortunately, aerobic microbes can flourish, which can lead to a decrease in oxygen in the water.

Anaerobic microbes, such as non-sulfur bacteria, are slower workers but do not require carbon dosing because they live in deep sand where organic matter is abundant.

The perfect storm for aquarium owners is to create a microbial community that is diverse and varied, which may mean the addition of a deep sand substrate, regular feeding with bacterial food and supplements, and the use of bacterial inoculants.

In conclusion

I hope you have enjoyed our tips on how to deal with excess nitrates in your fish tank.

Although nitrate is not as dangerous to fish as ammonia and nitrites, excessive levels of nitrate in the aquarium can lead to poor fish health, poisoning and shock. There are many ways you can prevent nitrates from building up in your tank, but the most effective method is to perform regular partial water changes and keep your aquarium clean and in good condition.

We’d love to hear what you thought of our article, as well as any tips and tricks you have for keeping nitrate levels low in your tank. Tell us your story in the comment box below and don’t forget to share our guide with your friends.

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