Aquarium Nitrate vs. Nitrite: Differences, Toxicity, & More

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Many aquarists know that nitrates and nitrites are bad for an aquarium, but not everyone knows the differences between the two. As I asked myself these questions many times before, I decided to gather all the relevant information into one article.

These are the general differences between nitrates and nitrites in aquariums:

Chemical formulaNO3NO2
Made fromAmmoniaNitrate
Acceptable levels10-20ppm0ppm
Can be toxic?YesYes

As we move forward, I will elaborate on the differences between nitrates and nitrites, and discuss their impact on aquarium fish. Then, I will share my tips on what you should do if their levels go higher than the recommended range.

An illustration of the nitrogen cycle that usually takes place in a healthy aquarium environment.

What Are The Differences Between Aquarium Nitrate & Nitrite?

People use the terms ‘Nitrates’ and ‘Nitrites’ interchangeably, and for a good reason. These substances are closely related. 

First of all, they are both forms of nitrogen. They appear when bacteria consume nitrogen. Although, the substances can also occur naturally in water, soil, and plants.[1]

If nitrate and nitrite levels spike whenever you perform a water change, your water source is probably contaminated by nitrates and nitrites. 

You can trace both toxins to the same sources, including fish waste, rotting leftovers, and decomposing plant matter. These are also the sources associated with ammonia.

In fact, you can use the same methods to combat nitrates and nitrites. That includes the installation of mechanical and biological filtration systems and the performance of routine water changes.

Some aquarists respond to high nitrate levels by adding more plants because aquatic plants can combat high nitrate levels by consuming the substance. 

They don’t realize that plants have a similar relationship with nitrites. Aquatic plants will reduce the nitrite concentration by absorbing the toxin. They use it as a building block.[2]

As you can see, nitrates and nitrites have a lot in common. The differences appear when you take a closer look at their chemical profiles, for instance:

  • Nitrates have one nitrogen atom and three oxygen atoms as opposed to the one nitrogen and two oxygen atoms you find in nitrites.[3]
  • Nitrites form when Nitrosomonas bacteria process ammonia. On the other hand, nitrates are the result of Nitrobacter feeding on nitrites.[4]
  • Nitrates are represented by ‘NO3’ while nitrites are ‘NO2.’

Most aquarists are familiar with the conversion of nitrites into nitrates. They don’t realize that nitrates can also become nitrites. 

Although, that tends to happen in human beings when enzymes in the mouth and body act upon the nitrates.[5]

Dirty fish tanks typically contain a lot of fish waste that builds up ammonia, nitrates, and nitrites.

Is Nitrate Or Nitrite Worse In Aquariums?

If you read the paper by Claude E Boyd dissecting nitrite toxicity (Department of Fisheries and Allied Aquacultures, Auburn University), you may conclude that nitrites and nitrates are equally dangerous.[6]

After all, the paper connects nitrites to lesions on the gills and skeletal muscle edema. It also explains what happens when the substance enters the bloodstream.

The nitrites bind to hemoglobin to form methemoglobin, and unfortunately, methemoglobin cannot combine with oxygen. 

This interferes with the body’s ability to transport oxygen. Obviously, that sounds terrible. The affected fish will eventually die.

However, if you look at the information this study (Nitrate Toxicity to Aquatic Animals) in the National Library of Medicine has provided, you can see that nitrates are just as destructive.[7]

After all, they also prevent oxygen-carrying pigments from carrying oxygen. Nitrate poisoning will cause infertility, damage to kidneys, lower immunity, and more.

Given enough time, nitrates will kill every fish in your aquarium, directly or through the stress they typically generate.

Does this mean nitrates and nitrites are equally dangerous? No, that conclusion is not correct. There’s a reason why nitrates in the human body don’t concern medical experts until enzymes convert them into nitrites.

Fortunately, the percentage of nitrates in the human body that become nitrites is low (5-10 percent).[8]

To the average fish, nitrates are just as dangerous as nitrites, but only in large concentrations. 

Fish can thrive in tanks with low nitrate levels. On the other hand, low nitrite levels will kill your tank’s inhabitants, lower their lifespan, or ruin their health. 

Simply put, the concentration is the most significant difference between the toxicity of nitrates and nitrites. 

This is why professional aquarists warn specifically against high nitrates levels in an aquarium instead of nitrates in general.

Vacuuming the aquarium’s gravel is essential when fighting water toxins, such as nitrates and nitrites.

What Comes First: Nitrite Or Nitrate?

You can debate this issue where people are concerned because the bacteria in enzymes can turn nitrates into nitrites. However, in aquariums, this conversion occurs in one direction. 

A paper from the US Environmental Protection Agency describes a process called nitrification in which a microbial procedure turns ammonia into nitrites and nitrates.[9]

Aquarists use the term ‘Nitrogen Cycle’ to refer to this operation. A new aquarium begins with ammonia, which you can introduce directly using commercial products or via fish waste and leftovers.

The purpose of adding ammonia is to nurture bacteria that feed on the substance, producing nitrites. 

If you succeed, ammonia levels will drop while the nitrite concentration rises. But at this point, you’re still not out of the woods. 

The objective of initiating the nitrogen cycle is to eliminate ammonia, which is toxic to fish. And unfortunately, while nitrites are less dangerous than ammonia, they are still poisonous to aquatic creatures.

Therefore, the nitrogen cycle must continue. Nitrosomonas bacteria feed on ammonia to make nitrites.[10]

Then Nictrobacter enters the picture, turning nitrites into nitrates, the least toxic option among these three substances. Therefore, in an aquarium, nitrites will come first. 

However, you can introduce nitrates to the tank in the absence of ammonia or even nitrites by adding water from a source contaminated by nitrates

Nitrates can enter local water bodies via septic systems, fertilizers, contaminated irrigation water from farms, etc.[11]

What Are Acceptable Nitrate And Nitrite Levels?

Humans are larger and more robust than fish. In that regard, you would expect the human body to do a better job of processing and withstanding nitrates and nitrites than fish.

And while that is true, the recommended nitrite and nitrate levels for fish and humans do not differ as drastically as you might expect. 

The Water Systems Council in the US wants to limit the concentration of nitrates in drinking water to 10 parts per million. 

On the other hand, the nitrites shouldn’t exceed 1ppm. That is also true for ammonia, which should be kept at 0ppm.

The organization based these figures on the EPA’s estimates regarding the level of nitrates and nitrites it takes to cause harm. 

In aquariums, you should keep nitrite levels at zero because it only takes 1ppm of the substance to kill your tank’s inhabitants. But fish can tolerate 10ppm of nitrates.

In fact, some fish can tolerate 50ppm of nitrates. Although, you are better off keeping the concentration below 25ppm.[12]

What Should I Do With Elevated Nitrites & Nitrates?

As you probably know, you can test your aquarium water for nitrates, nitrties, and ammonia using a test kit. Here is an article where I explained how to do that.

If you don’t own one, I personally suggest getting the API FRESHWATER MASTER TEST KIT (link to Amazon). This kit lasts for about eight hundred measures, making it pretty cost-effective.

If you found that the ammonia and nitrite are above 0ppm, or that the nitrates are higher than 10-20ppm, the first step would be performing a large water change.

I typically suggest changing 20 to 50 percent, depending on the severity of your situation. Either way, it should be more than what you’re replacing weekly.

Then, focus on cleaning your aquarium from debris and leftovers. The most important step is to siphon the substrate using an aquarium vacuum.

I use the Laifoo Aquarium Siphon Vacuum Cleaner (link to Amazon) for this purpose, but frankly, almost any vacuum cleaner will do. 

If you’ve never done this before, here is an excellent Youtube video that will walk you through the entire process:

After making a large water change and cleaning your tank, the next step is to check your water source. You can use the same kit you used to test the water in your tank.

Even if everything is okay with the new water, I still recommend using a water conditioner, just to be on the safe side.

A water conditioner wouldn’t remove ammonia. However, it will efficiently bind to the toxin and neutralize it. The same goes for nitrates and nitrites.

One of the best conditioners for this purpose is the well-known Seachem Prime Fresh and Saltwater Conditioner (link to Amazon).

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.

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Both nitrates and nitrites are prevalent in home aquariums, and even though they sound similar, there are considerable differences between the two.

In the process known as the nitrogen cycle, nitrifying bacteria are responsible for turning ammonia into nitrate, an element consisting of three oxygen atoms and a single nitrogen atom (NO3).

Then, similar bacteria turn nitrates into nitrites, which consist of two oxygen atoms and one nitrogen atom (NO2).

Both substances are toxic to aquarium fish but at different concentrations. While nitrites can be dangerous from 1ppm, nitrates are more tolerated and considered toxic from 10 to 20ppm.