Aquarium Nitrate 101: Causes, Solutions, Levels & More

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We all know that nitrate can be bad for fish. Unfortunately, that was all that I knew when I started this hobby.

I had no idea what causes nitrate, what levels of this toxin are acceptable, or what steps I should take to get rid of it.

So, to save you a lot of time and trouble, I decided to gather all the essential information into one article.

What Causes High Nitrates In Aquariums?

Nitrate is formed from the metabolism of ammonia. Therefore, anything that leads to ammonia can potentially cause high nitrates in aquariums. That typically includes rotten fish food, waste, and dead organisms.

However, unlike ammonia, nitrate can be brought from the water source itself. While it is pretty rare for tap water to contain ammonia, in many cases, some traces of nitrate can be found.

That is why so many households use different filters to remove nitrates, such as reverse osmosis and ion exchange.

Dirty fish tanks are a significant risk factor for toxins, including ammonia, nitrite, and nitrate.

Sounds interesting? Here is an article where I talked about all the causes of high nitrate in aquariums. I also mentioned some useful tips on how to prevent nitrates from going up.

How Do I Test For Aquarium Nitrate?

Most fish owners use testing kits to measure the nitrate concentration in their tanks. The average kit will also measure the ammonia, nitrite, and pH levels.

When choosing a testing kit, it is better to use liquid ones. Those usually contain a unique solution that changes its color when reacting with toxins like nitrate.

Testing strips are usually cheaper, although they are far less accurate. Either way, do not test the water immediately after a water change.

You will achieve more accurate results if you wait for 10 to 15 minutes, as you need the water to mix evenly through the tank.

Still curious? Here you will find all the information on how to test for aquarium nitrate. I also shared a pretty useful video for a better visual understanding.

How Do I Get Rid Of Nitrates Fast?

If you need an urgent solution for your nitrate situation, you have two courses of action:

1. Make A Partial Water Change

A water change is the most obvious solution to fight high nitrate levels. If the nitrates in your tank exceed 40 to 50 ppm, you need to change at least 15 to 20 percent.

The amount of water you replace mainly depends on your tank’s size and the condition of your aquarium.

The downside with a water change is that sometimes the water source itself contains nitrates. That is why I usually suggest testing the water beforehand.

2. Get A Water Conditioner

Using a water conditioner is the fastest way to fight ammonia, nitrite, and nitrate. It is the perfect solution when your fish are in distress and you have to act quickly.

The one that I use is the well-known Seachem Prime Conditioner (link to Amazon). It is suitable for both fresh and saltwater tanks.

All you have to do is to use 1 capful (5 mL) for every 50 gallons (200 L) of new or replacement water.

Water conditioners, including the Seachem Prime conditioner, typically bind and neutralize the ammonia. And if there is no active ammonia, nitrate will not form.

How Do I Get Nitrates Out Naturally?

The easiest way to lower nitrate levels is to perform a water change. You can perform smaller water changes daily or one large water change weekly.

But what if water changes are not an option? You can experiment with water conditioners. But if you prefer natural solutions to your nitrate problem, consider the following:

1. Microbes & Biopellet Reactors

Some aquarists don’t expect bacteria to play a significant role in removing nitrates. After all, bacteria only matter when ammonia and nitrites are present. 

They turn ammonia into nitrites and nitrites into nitrates.[1] Once the ammonia and nitrites are gone, Nitrosomonas and Nitrobacter don’t serve a purpose.

You must nurture them in the filter media to prevent ammonia and nitrite levels from spiking. However, they can’t consume or process nitrates.

Fortunately, Nitrosomonas and Nitrobacter are not the only microorganisms at your disposal. Many aquarists nurture aerobic and anaerobic microbes that turn nitrates into nitrogen gas. 

Nitrogen gas is not a threat because it usually evaporates. Aerobic microbes are somewhat challenging because they can’t perform their role unless you feed them carbon. 

They can also run amok, multiplying fast enough to deplete the oxygen. Anaerobic microbes don’t require carbon to process nitrates. However, they work at a slower pace.

Between the two, anaerobic microbes are the least dangerous because they won’t bloom.[2] As such, oxygen deficiencies are unlikely to occur.

Don’t expect these organisms to occur naturally in the aquarium. Talk to your local fish retailer and ask them to find some live cultures for you. They can also recommend some reputable biopellets. 

Biodegradable pellets are all the rage these days because they contain beneficial bacteria. 

Using a biopellet reactor, you can add beneficial bacteria to your tank by passing water through the biopellets. Biopellets and protein skimmers go hand-in-hand.[3]

If you’re hesitant to experiment with biopellets, consult other aquarists. Look for established tanks with thriving microbial communities and transfer their substrate or filter media to your aquarium. 

These microbes are similar to nitrification bacteria (Nitrosomonas and Nitrobacter); that is to say, if you can introduce them to your tank as early as possible, they will keep nitrate levels under control in the long run. 

Experts from Portugal (Departamento de Zoologia-Antropologia, Faculdade de Ciencias da Universidade do Porto) performed a study investigating the nitrate removal capacity of denitrifying bacteria in marine aquaculture systems.[4]

They proved that these microbes can work as effectively in saltwater aquariums as they do in freshwater tanks.

2. Growing Aquarium Plants

Many amateur aquarists are unaware of the close relationship between plants and nitrates.

They don’t realize that nitrates are food to plants which, in turn, makes plants the perfect solution to high nitrate levels.

If you have a planted tank and the nitrate concentration keeps falling even though you haven’t performed any water changes, the plants are at fault. 

But you should keep two significant factors in mind. First of all, plants prefer ammonium to nitrates.

A study in ‘Ecology of the Planted Aquarium’ exposed Elodea nuttallii to ammonium and nitrates. After 16 hours, the plant consumed 75 percent of the ammonium. However, it had barely touched the nitrates.[5]

If your tank has both ammonium and nitrates, the plants will only consume the nitrates once the ammonium is depleted.[6]

If you have a decent plant population but nitrate levels are still high, test for ammonium. But ammonium is not your only concern. 

Plants require light to consume nitrates. You can expect plants in dark environments to take up fewer nitrates than plants in lit conditions.

This is another factor that elevates ammonium over nitrates. The presence or absence of light doesn’t affect a plant’s ability to consume ammonium.

Ultimately, even with their penchant for choosing ammonium over nitrates, plants are still an effective nitrate remover.

A paper in Biogeochemistry investigating the impact of vegetation on denitrification noticed that macrophytes (macroscopic aquatic plants) competed with denitrifying bacteria for nitrates.[7]

Plants can work with aerobic and anaerobic microbes to keep nitrates below the appropriate threshold.

Aquarium plants will consume nitrate and remove it naturally, as well as ammonia and nitrite.

3. Using Aquarium Algae

Many aquarists have been taught to perceive algae as the enemy. As such, the notion of using algae to fight high nitrate levels surprises them.

And yet, a paper in Applied and Environmental Microbiology compared the nitrate uptake rates of various algae species, proving that algae are a potent answer to high nitrate levels.[8]

You also have a paper in the ‘Water, Air, and Soil Pollution’ journal that advocated for the use of immobilized heterotrophic algae in the removal of nitrates from groundwater.[9]

And it isn’t the only one.

A study in Current Biotechnology (Stephen R. Lyon, Mahboobeh, Marcia A Murry, and Hossein Ahmadzadeh) that wanted to summarize nitrate removal rates showed that algae could remove ammonia, nitrates, and nitrites from water.[10]

These studies will put to rest any doubts you have about algae’s ability to remove nitrates. However, algae are tricky.

People approach it with caution because algae is still a threat to aquariums. It can destroy your plants by consuming valuable nutrients.  

The key is to prevent algae from growing uncontrollably by adding bacteria that consume the nutrients the organisms use to grow. 

You can also add species like mollies and snails that eat algae, not to mention improving water quality by creating and following a strict maintenance routine.

Algae are a menace, but they serve a purpose. The organism can eliminate various toxic substances, including ammonia and phosphorous.

Algae only become a challenge when you allow the organism’s growth to go unchecked. 

Interestingly enough, the more algae you have, the more nitrates it will consume. And the more nitrates it consumes, the faster the algae will grow. 

But the last thing you want is for algae to overrun your aquarium. Don’t allow the algae to grow uncontrollably just because it is your only nitrate removal tool. 

Use a combination of methods, including a moderate amount of algae.

Growing aquarium algae is an excellent, natural way to fight toxins like nitrate.

4. Reducing The Fish Population

How many fish do you have in the tank? Overcrowding encourages nitrate spikes because fish choke the water with waste.

That waste produces ammonia which becomes nitrites, and finally, nitrates because of the actions of the nitrifying bacteria.

The more fish you have in the tank, the more waste you can expect, and the higher the concentration of nitrates.

Plants and algae won’t help you if nitrifying bacteria produce nitrates faster than they can consume. 

To prevent the nitrate levels from spiking incessantly, start by cutting the fish population.

Overcrowding is a recipe for disaster, associated with stress among aquatic creatures and oxygen deficiencies. If the nitrate levels rise, even as the oxygen content falls, your fish will die. 

Reducing the fish population prevents unexpected nitrate spikes, giving the plants, algae, and microbes a chance to lower the nitrate concentration.

Use the ‘One inch of fish per gallon’ rule.[11] For example, a two-inches molly will require two gallons, two mollies of that size will need four gallons, and so on.

It is pretty common to see an ammonia and nitrite spike in overpopulated fish tanks.

5. Cutting Down The Meals

Overfeeding is just as dangerous as overcrowding. Overfeeding introduces two challenges. First, the fish will generate more waste. Secondly, the leftovers will increase. 

Don’t expect fish to eat every morsel you add to the tank. The fish will eat what they can and ignore the rest. Those leftovers will float to the bottom and rot unless you remove them.

If you overfeed the fish, the leftovers will increase. This will lead to a spike in the concentration of ammonia.

Because nitrates manifest when bacteria act upon ammonia and nitrites, a spike in ammonia will inevitably produce a spike in the nitrate levels.

The reverse is also true. You can keep the nitrate concentration down by preventing ammonia levels from spiking.

One way of doing this is to avoid overfeeding. Add food in amounts the fish can finish in five minutes or less. You can also introduce creatures such as snails that eat leftovers. 

6. Maintaining A Clean Tank

Water changes are not the only way to keep the aquarium clean. Every tank requires a filter. Make sure yours is working. 

A dead filter allows the water to stagnate, introducing oxygen deficiencies. Make sure the filter’s strength matches the tank size.

If the filter’s strength matches the tank’s size, all the water will pass through the filter four times an hour (minimum). 

But a filter can’t do all the work. Use a net to remove any solid debris you see, including dead plants and animals.

Even if you’re too smart to overfeed your fish, leftovers will still accumulate in the aquarium. For that reason, you can’t forget to vacuum the substrate.

A clean tank prevents nitrate levels from rising. Strict maintenance routines can also keep the algae population under control. 

Plants, algae, and microbes can keep the nitrates below the recommended threshold in a clean, well-maintained tank.

Some people use their algae population to measure the cleanliness of their tank. They know that algae thrive in water with a high nitrate content. 

In other words, an algae bloom is almost always a sign of trouble. People with cloudy tanks are encouraged to test for nitrogenous compounds. 

Vacuuming the aquarium substrate is mandatory as it typically contains rotten leftovers and debris.

How Long Does It Take For Nitrate Levels To Go Down?

Here is a table summarizing the time it takes for nitrate to go down, depending on the method you choose:

Water conditionersA few minutes
A water changeAs soon as it’s done
Nitrifying bacteriasix weeks

As you can see, the fastest way to remove nitrate from aquariums is with a water conditioner. That will work within minutes.

Even though some conditioners don’t neutralize nitrate directly, most of them will neutralize ammonia, preventing nitrate from forming down the line.

Using a water conditioner is an excellent way to prepare the tank for a significant water change. 

Do not rely solely on water conditioners. You still have to change the water and remove what spiked the toxins in the first place.

Sounds interesting? Here you can learn more about how long it takes for nitrate to go down. I also discussed whether nitrate is expected to go down on its own.

What Are The Differences Between Nitrate & Nitrite?

These are the main differences between nitrate and nitrite:

  • Nitrate is made up of a nitrogen atom and three oxygen atoms (NO3), while nitrite is consist of a nitrogen atom and two oxygen atoms (NO2).
  • In aquariums, nitrate is formed from ammonia, while nitrite is made from nitrate.
  • Acceptable levels of nitrate are 10 to 20 ppm. However, nitrites shouldn’t exceed 0 ppm in home aquariums.

While nitrate and nitrite are two different substances, they are both toxic to aquarium fish and should be treated when going out of control.

You can expect these two substances to form in overcrowded and under-maintained tanks, as they are both byproducts of ammonia.

Caught your attention? Here is an article where I elaborated on the differences between nitrite and nitrate. I also shared a useful video on what to do when these two start to accumulate.

Why Are Nitrates High After A Water Change?

Nitrate may remain high after a water change when the water source itself contains the substance. However, it is also likely that you haven’t replaced a sufficient amount of water.

As many water sources contain toxins like chlorine, chloramine, nitrate, and heavy metals, I typically encourage aquarists to test the water beforehand.

You can use the same testing kit you are using for your aquarium water. If toxins are detected, apply a water conditioner or consider using another source.

If everything is fine with the water source, make sure you wait at least 10 minutes before testing. If you test the water immediately after the change, you will get misleading results.

Also, in most cases, you should change at least 10-15 percent to see a difference. That is especially true for tanks larger than 20 gallons.

Testing the tap water for toxins is a crucial step before making a water change in your tank.

Wish to learn more? Here I explained all the reasons why nitrates remain high after a water change. I also explained how the testing kit itself can be unreliable.

What Are The Signs Of High Nitrates In Aquariums?

These are usually associated with high nitrates in fish tanks:

  • You’ll notice excessive algae growth.
  • Your fish will die for no apparent reason.
  • Juvenile fish will stop growing properly.
  • Some fish will become lethargic and lose their appetite.
  • The aquarium water will be cloudy.

If you notice some of the signs above, it is time to check the water parameters in your tank, as these are also associated with elevated ammonia.

The earliest sign is the behavioral changes in your fish. Some of them will stay at the bottom of the tank motionlessly, while others will stop eating altogether.

Sounds interesting? Here you will find more information on the signs of high nitrates in aquariums, including some other signs fish might manifest.


Nitrate can be a bad sign when detected in home aquariums, as it usually means that other toxins, including ammonia and nitrite, are elevated as well.

Yet, some concentrations of nitrate are acceptable. This toxin only becomes dangerous when reaching levels of 40 to 50 ppm.

That is unlike ammonia and nitrite, which are toxic from anything above 0 ppm. Either way, when detecting high levels of these toxins, it is best to apply a water conditioner.

Then, replace at least 10 to 15 percent of the water. Just make sure to test the water source beforehand, as it might be contaminated as well.

Pro tip: Dealing with ammonia is key when you find high nitrate levels in your tank. Here is an article with all the information you need about ammonia, including the specific products I use to remove it.