Nitrates vs Nitrites – How to Balance an Aquarium Setting

Alison Page

Alison Page


Nitrates vs Nitrites Aquarium

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What are nitrates vs. nitrites in the aquarium? Why is it important to know how to manage their levels? Are these chemicals dangerous to your fish?

So, there’s some science involved in the aquarium hobby, but understanding how your fish tank’s ecosystem works needn’t be complicated. However, you need to understand the difference between nitrates and nitrites and how both chemicals play a role in freshwater and saltwater aquariums.

Keep reading to find out everything you need to know about nitrates and nitrites and why aquarium water chemistry is so important for your fish’s health and longevity.

What Is the Aquarium Nitrogen Cycle?

If you’ve ever had an aquarium or are just starting your first one, you’ll most likely have heard the term nitrogen cycle. Ensuring your tank is what’s termed in the hobby fully cycled before introducing any fish is essential for a happy and healthy fish tank.

So, what is the nitrogen cycle, and how does it relate to nitrites and nitrates in your aquarium?

Nitrogen Cycle Overview

In the natural environment, the nitrogen cycle describes the process of nitrogen moving from the air to animals, plants, and bacteria and then back into the air. That’s an incredibly efficient process that needs no help from us. However, the nitrogen cycle works somewhat differently in an enclosed fish tank environment.

In your fish tank, fish waste, dead plants, and other organic matter continually decompose, forming the chemicals ammonia, nitrites, and nitrates. In the final phase of that cycle, nitrates are absorbed by living plants in your tank for use as nutrients or are removed from the water by your biological filter system, and partial water changes.

Ammonia to Nitrites to Nitrates

So, how does the decomposition process work?


When you have fish and other living organisms in your aquarium, they produce waste throughout the day and night. In addition to that, dead plant matter, uneaten fish food, and dead organisms accumulate at the bottom of the tank.

Various species of bacteria feed on that organic waste, causing it to break down, ultimately producing ammonia. Ammonia is highly toxic to fish and invertebrates. Even low levels of ammonia in the aquarium water will burn the fish’s gills and cause oxygen deprivation. Very high ammonia levels usually happen when the tank is overcrowded, the fish are overfed, or the aquarium has not been cycled properly.

Ammonia to Nitrite

When the aquarium is balanced, the nitrogen-fixing bacteria, Nitrosomonas, consume the ammonia, converting it to nitrite.

Nitrite is toxic and disrupts the oxygen-carrying abilities of blood. However, fish can generally cope with twice as much nitrite in the water as ammonia.

Nitrite to Nitrate

In the next stage of the nitrogen cycle, Nitrobacter bacteria process the nitrites into a less toxic chemical called nitrate. Nitrates are not toxic at low levels, but if the concentration in the water rises above 20 ppm, the environment can become dangerous to your fish.

Nitrate converts aerobically in nature into harmless nitrogen gas. However, that can’t happen in the enclosed environment of your fish tank, so you must carry out partial water changes to dilute the chemical.

Plenty of living plants are essential in a freshwater tank since they take up nitrates from the water for use as food. In a saltwater tank, live rock and a deep sand substrate form anaerobic areas where bacteria can turn nitrates into harmless nitrogen gas, which simply evaporates.

What Is a Suitable Nitrite Level in an Aquarium?

Ideally, nitrite levels in an aquarium should be zero.

Nitrite is a deadly form of nitrogen that can quickly lead to nitrite poisoning in fish and could easily wipe out your entire collection! High nitrite levels can lead to nitrite poisoning, where nitrite binds with hemoglobin in the fish’s bloodstream, leading to suffocation.

So, it’s essential that you remove any nitrites from your tank water as soon as possible. Test the water weekly to ensure the levels are zero, and carry out a water change to correct any imbalances.

What Is a Suitable Nitrate Level for a Fish Tank?

Unlike ammonia or nitrite, nitrate isn’t immediately toxic to fish and invertebrates. However, you still want the levels to be as close to zero as possible.

How many nitrates you have in your aquarium largely depends on your tank’s inhabitants and maintenance schedule, and most tap water contains nitrates at low levels. Hence, a zero level is extremely difficult to achieve. Unlike ammonia or nitrite, nitrate cannot be processed by beneficial bacteria in the aquarium and needs to be processed by other organisms or manually removed.

Most freshwater aquariums can do fine with under 30 ppm nitrates, especially if you have a planted aquarium. Live plants take up all forms of nitrogen, from ammonia to nitrate, making them great for keeping the tank clean and healthy for your fish.

Saltwater aquariums are more sensitive to poor water quality, though most hobbyists still allow up to 20 ppm of nitrate even in their high-tech reef setups.

Whatever the threshold for your aquarium is, nitrate will need to be controlled through regular water changes. Excess levels, usually above 40 ppm, can start to cause stress in fish and even die-off of invertebrates.


In this part of our guide, we answer some of the most frequently asked questions about nitrates and nitrites in the aquarium.

Q: Do Aquarium Plants Absorb Nitrates or Nitrites?

A: Aquarium plants absorb both nitrates and nitrites as a source of nutrition. Plants need nitrogen to promote healthy growth, taking up what they need from the water column and aquarium soil through their roots.

Nitrate is the predominant form of available nitrogen in the environment since it’s formed during the final part of the nitrogen cycle. Therefore, nitrates are more plentiful than ammonia or nitrites in a fully-cycled aquarium.

Generally, fast-growing plant species like Pogestemon stellatus and Water Sprite use nitrates more quickly than slower-growing plants like Java fern and Anubias.

Q: How Do I Lower Nitrite and Nitrate Levels in My Aquarium?

A: The quickest way to lower nitrite and nitrate levels in your fish tank is to perform a partial water change.

You should change around 30% of the water at first and then test the water using a high-quality aquarium water testing kit. If the levels are still too high, repeat the exercise. However, don’t change too much water at once, or you risk removing the beneficial bacteria that process the ammonia and nitrites, potentially causing a disastrous ammonia spike.

Continue carrying out 30% weekly partial water changes, and maintain your filtration system correctly, as explained earlier in this guide.

Q: What Is a Good Nitrite Level for an Aquarium?

A: Ideally, the nitrite level in your aquarium should be zero, as even small levels of this chemical are potentially fatal to your fish.

Q: What Comes First, Nitrite or Nitrate?

A: Ammonia is the first chemical that’s produced in your aquarium, given off by decomposing fish waste and other organic matter. The ammonia is then converted into nitrite by Nitrosomas bacteria, and the nitrite is converted into nitrates by Nitrobacter bacteria.

Q: What Kills Fish, Nitrate or Nitrite?

A: Nitrite is extremely toxic to fish; even a small amount in the water will kill them quickly. Nitrates are less harmful to your livestock, although high levels can be deadly, so keeping the chemical to a minimum in the water is essential.

Final Thoughts

When it comes to nitrates vs. nitrites in the aquarium, nitrites are the most harmful. Nitrites at relatively low levels are deadly to your fish! Nitrates can be dangerous, too, but only when levels are very high.

The best way to quickly reduce nitrites and nitrates in the aquarium is by performing partial water changes. You must also install an efficient filtration system with a biological filter where bacteria can process the ammonia, nitrites, and nitrates produced during the nitrogen cycle.

Living plants are great at taking up harmful nitrites and nitrates from the water to use as a nutrient source, helping to keep the aquarium water clean and safe for your fish. So, be sure to include plenty of plants of different species in your setup!

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