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What Should The Nitrate Level Be In A Fish Tank?

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I remember how every time I tested my aquarium water, I kept asking myself if the nitrates were too high. As I knew they have different toxicity than ammonia and nitrites, I kept feeling confused when I got results like 10, 20, and 30ppm.

Ideally, aquarium water nitrate should be kept under 20ppm. Nitrates are potentially toxic like ammonia and nitrite. However, while ammonia is toxic from anything above 0ppm, nitrates only become dangerous at 40-50ppm and above.

As we move forward, I will explain what could happen if the nitrates are too high, and what steps you should take if it happens in your tank. You will also learn how to prevent this from happening in the future.

Bacteria that turn nitrite into nitrate typically sit and flourish within the filter media.

What Should The Nitrate Levels Be In A Fish Tank?

Nitrates are technically a good thing. They appear when bacteria consume ammonia and nitrites in a process called the nitrogen cycle.

Ammonia and nitrites are toxic. Therefore, you want bacteria to attack these substances. Nitrates are supposed to be harmless.

And yet, you have experts from Huazhong Agricultural University, China University of Geosciences, and Hubei Provincial Engineering Laboratory performing studies to identify effective nitrate removal methods.[1]

This doesn’t necessarily make nitrates toxic. Or, at the very least, they are not as dangerous as ammonia and nitrites. Nitrates are harmless in small doses. They only become problematic in high concentrations.

But the definition of ‘High Concentration’ will vary depending on who you consult. Although, many aquarists agree that 10ppm is the safest bet. 25-50ppm is also acceptable. You have more room to maneuver with nitrates. 

Ammonia and nitrites are so dangerous that aquarists endeavor to keep their concentrations at zero

On the other hand, your fish can survive in a tank with nitrates if you keep the concentration below 50ppm.[2]

On that matter, feel free to check this article, where I elaborated on the differences between nitrates and nitrites. I also shared the recommended levels for each one.

You should also keep algae in mind, as elevated nitrate concentrations promote algae growth.

Many people keep nitrate levels below 10ppm even though fish can tolerate amounts as high as 100ppm because higher nitrate levels encourage algae growth.

The last thing you want is to choke your aquarium with algae. Dense algae populations will destabilize your water’s chemical balance. For that reason, some aquarists prefer to keep the nitrate concentration at zero.

But maintaining zero nitrates is easier said than done, especially when the substance has so many sources. Although the objective is understandable. 

Fish are not your only concern. Even in the absence of algae, different creatures have different levels of tolerance where nitrates are concerned. 

For instance, invertebrates may wither in water with nitrate levels that allow fish to thrive.

An illustration of the nitrogen cycle, describing the process toxins typically go through in a fish tank.

What Level Of Nitrate Is Toxic To Fish?

This question doesn’t have easy answers. Some people think that nitrate levels above 100ppm are too dangerous. Others classify nitrates as non-toxic at 200mg/L.[3] For the record, 1mg/L = 1ppm.[4]

It is also worth noting that nitrate sensitivity varies. Some people raise fish in tanks with nitrate levels as high as 200ppm, possibly even higher. 

But then, you have species like discus that don’t appreciate nitrate levels higher than 20ppm. This doesn’t even account for the impact of chronic exposure to nitrates. 

John Davidson and a team of scientists from the Freshwater Institute (Conservation Fund) studied the harmful consequences of nitrate exposure among rainbow trout.[5]

Even though they performed their study in environments where the maximum recommended nitrate level was 1000 mg/L, Davidson’s team observed physical deformities in trout raised in aquaculture systems with nitrate levels as low as 75 mg/L.[6]

What does this mean? There is no definitive toxic level for nitrates. Some fish species can thrive in nitrate levels that typically kill other fish species.

Don’t expect every creature in the aquarium to manifest a predictable reaction to the same nitrate concentration. Your best option is to keep the nitrate levels at 10ppm. 

If 10ppm is not feasible because of the type of fish you rear or the fertilizers your plants require, don’t exceed 50ppm.

Nitrate levels below 100ppm are technically acceptable in a community tank. But you are better off playing it safe. Stick to 50ppm or less.

What Happens If The Nitrate Levels Are Too High?

Nitrates are harmless in small doses. But if you want to know what they do in high concentrations, consider their impact on human health. 

A paper analyzing nitrate levels in groundwater in Southern Iran (Shiraz University of Medical Sciences, Behbahan Faculty of Medical Sciences, Ahvaz Jundishapur University of Medical Sciences) connected nitrates in drinking water to involuntary abortions, infertility, and infant methemoglobinemia.[7]

If high nitrate levels are so dangerous to humans, what do you think they will do to fish? You already know that they cause deformities in rainbow trout. 

But that is not all. A paper in Conservation Physiology blamed nitrates on a reduction in the blood oxygen-carrying capacity of freshwater fish.[8]

Although, the paper also noted that elevated nitrates worked in tandem with an acidic pH to lower the oxygen-carrying capacity of aquatic creatures.

Another paper in aquaculture engineering (John Davidson, Steven Summerfelt, Christopher Good, and Carla Welsh) noticed that fish exposed to 100 mg/L of nitrates manifested strange and erratic swimming behavior.[9]

An article from Reviews in Aquaculture found that high nitrate levels reduced reproductive performance.[10] Suffice it to say, high nitrate levels are not good. 

Expect all the standard signs of stress, including listlessness, loss of appetite, labored breathing, and more. Don’t be surprised if fish in tanks with high nitrate concentrations become violent. 

Some fish respond to toxins by becoming less active. Others display signs of aggression that may contradict a typically peaceful temperament.

To prevent this from happening, I usually recommend testing the aquarium water every week. It is also crucial to test the water source itself.

Elevated toxins like nitrates and ammonia may compel aquarium fish to hide at the bottom.

Can Fish Survive High Nitrates?

It depends on the fish. As you know, nitrates that are too high for one species might be tolerable for another species. 

This review in the National Library of Medicine has highlighted the fact that freshwater creatures are more sensitive to nitrates than their marine counterparts.[11]

If nitrite levels exceed a fish’s tolerance, it will die. This is true for every toxic substance. The rate at which the fish dies depends on the concentration. 

Some fish will die within minutes or hours if you add them to water with nitrate levels of 100ppm. 

Others will display concerning signs such as loss of appetite and lethargy, which are also seen when ammonia is elevated.

But they may linger for days, possibly even weeks, before dying. You also have species that can survive at concentrations as high as 100ppm.

They will do so miserably, displaying every problematic symptom you can imagine. They may even develop skeletal deformities. 

But don’t be surprised if they live a full life, a miserable life, but a full one all the same. Again, you can’t predict your fish’s response to nitrates. 

If you think your aquarium’s nitrate levels are acceptable, but your fish are sick and distressed, take that as a sign that those nitrate levels are too high for the species in the tank.

What Should I Do When The Nitrate Level Is Too High?

If the nitrates in your tank spiked, the ammonia is probably high as well. In this case, your first step is to perform a significant water change.

With nitrate levels above 20ppm and ammonia higher than 0ppm, it is best to replace roughly 50 percent of the aquarium water.

While there are several ways to remove nitrate, a decent water change is usually the cheapest and fastest solution. Just make sure to test the water source itself beforehand.

The next step would be to eliminate the source of that nitrate. Most commonly, you will find plenty of debris and leftovers hidden in the substrate.

When left for extended periods, organic matter will decompose and produce ammonia, which will, later on, turn into nitrite and nitrate.

That is why I usually recommend siphoning the substrate with products like the Laifoo Aquarium Siphon Vacuum Cleaner (link to Amazon).

You should also perform routine water changes as a preventive measure. I typically replace 10 to 15 percent in tanks larger than 20 gallons, and 20 to 25 percent in smaller ones.

Vacuuming the substrate is one of the most crucial steps when detecting elevated toxins in a fish tank.

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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Unlike ammonia and nitrite, fish can tolerate relatively high levels of nitrates. That doesn’t mean that the substance isn’t toxic. It only means that it becomes dangerous when reaching higher concentrations.

As a rule of thumb, it is best to keep nitrates under 20ppm. They don’t have to be strictly kept at 0ppm like ammonia.

However, if you measure levels of 40ppm or above, you should perform a water change. It is also essential to test the water source itself to see if it is contaminated.