Why Are Nitrates High After A Water Change?

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We all know that water changes are the answer to elevated toxins, including nitrates. But in some cases, even after a decent change, the nitrates remain high. As I suffered from this issue in the past, I decided to share all that I know about it.

Nitrates typically remain high when the water change was too small. The aquarium toxins won’t be diluted sufficiently if not enough water is added. However, in some cases, the water source itself contains nitrates.

As we move forward, I will share some other possibilities that might have caused your nitrates to remain high after a water change. I will also show you what is the right amount of water to change, and when exactly you should perform the test.

In some areas, tap water contains high nitrate levels that are toxic to aquarium fish.

Why Are Nitrates High After A Water Change?

Nitrates are only dangerous in high concentrations. More importantly, you lower nitrate levels by performing a water change. 

The last thing aquarists expect is for nitrate levels to remain high even after a water change. Fortunately, this phenomenon has very few causes. They include:

1. The Water Change Was Too Small 

How much water did you change? Water changes perform two tasks. First, they take some of the nitrates out of the tank. 

Secondly, they dilute the remaining nitrates with new water. The efficacy of the water change will depend on the size of the water change.

Aquarists will only perform massive water changes of 50 percent and above during emergencies when nitrate levels are so high they require drastic measures.

A 10 – 15 percent water change every week is more than adequate for most tanks. Although, it is not uncommon for aquarists to raise that figure to 25 percent.[1]

If nitrate levels are still too high despite the water change, maybe the water change is too small.

Perhaps the water change took some of the nitrates out, but it wasn’t enough to make a significant impact on the nitrate concentration.

You see this in neglected tanks where nitrate levels have been allowed to go unchecked for several days, possibly even weeks.

The nitrate levels are so high in some situations that daily water changes are required. 

Try performing a larger water change of 60 percent or more. If the testing kit reveals lower nitrate levels, your previous water changes were too small.

2. The Testing Kit Isn’t Reliable

A Thesis from the Graduate Faculty of Auburn University highlighted the critical role testing kits play in aquaculture.[2]

Aquarists use these tools to make crucial decisions even though the reliability of the data they generate is questionable, especially in comparison to standard testing methods. 

Amateur aquarists take the results their testing kits provide at face value, which is problematic because people buy defective tests all the time. 

Are you sure your nitrate levels are still high after the water change, or is your testing kit generating the wrong results? Don’t take any drastic steps without double-checking the results.

Buy another testing kit and check the water. Do the new results match the old results? Better yet, take a sample of your water to a lab. A lab will provide the most reliable results.

Don’t use your local fish store. They are not a suitable alternative to a lab. Your local store will probably use the same testing kits you have at home.

If your testing kit is the problem, abandon that particular brand, especially if multiple testing kits from the same brand provided the wrong results.

You should also avoid cheap testing kits. Look for reputable options that attract overwhelmingly positive reviews from consumers.

From my experience, the API FRESHWATER MASTER TEST KIT (link to Amazon) is one of the most reliable options. It also lasts for about eight hundred measures, making it highly cost-effective.

If you feel uncomfortable with those test kits, feel free to check my complete article on how to test for aquarium nitrate. I also discussed how often you should make those tests.

Vacuuming the substrate is a central part when trying to prevent the accumulation of aquarium toxins.

3. Poor Aquarium Maintenance

What kind of maintenance routine do you follow? Nitrates are leeching into your aquarium from somewhere. This substance forms when bacteria convert ammonia into nitrates.

Studies like the one the Natural Sciences and Engineering Research Council of Canada funded have gone to great lengths to identify the microorganisms responsible for this process.[3]

Don’t be surprised if your tank is also plagued by high ammonia and nitrite levels. Look for rotting organic matter in the water.

You should also rinse the filter media. The filter media is supposed to keep the water clean. But it will achieve the opposite result if you allow organic matter in the filter media to rot.

Another important step is to vacuum the aquarium’s substrate. You won’t believe how much organic matter is hidden in there.

I personally use the Laifoo Aquarium Siphon Vacuum Cleaner (link to Amazon) for that purpose. It is also efficient in preventing ammonia from building up.

An illustration of the nitrogen cycle that is typically found in a healthy home aquarium.

4. There Are Nitrates In The Water Source

Where did the new water originate? If you’re like most people, you use tap water because it is convenient.

Additionally, you probably applied de-chlorinators because you know that water suppliers add chlorine and chloramine to kill bacteria.

Those substances make the water safer for human consumption. Unfortunately, they kill fish. Aquarists use water conditioners to eliminate these toxins.

It doesn’t occur to them to test for nitrates because they don’t associate nitrates with tap water. But some regions have dangerously high nitrate levels.

If nitrate levels in your aquarium are always high after a water change, test the tap water. High concentrations as high as 40ppm should scare you.

Most organizations want nitrate levels in drinking water to stay below 10ppm.[4] Interestingly enough, that same threshold applies to fish. 

They can live with nitrates of 25 or even 50ppm, but 10ppm is the recommended amount.

A paper in the Journal of Environmental Health Science and Engineering explores the heterotrophic and autotrophic processes people can use to remove nitrates from water.[5]

But you don’t have to concern yourself with such complicated methods. Water conditioners are sufficient. 

Use these commercial products to treat tap water before adding it to the tank. Just make sure the water conditioners are designed to tackle nitrates.

5. The Nitrates Aren’t Necessarily High

As I previously discussed, ammonia must be kept at 0ppm. You can find the information here, where I talked all about the different ammonia levels. 

But that isn’t the case with nitrates. Fish can tolerate this substance until it reaches 20ppm. If your testing kit measures 5-10ppm of nitrates, it is still considered okay.

Bear in mind that drastic water changes can stress your fish even more than relatively low nitrate levels.

However, this course of thinking doesn’t apply if the levels are 20ppm or higher. That isn’t acceptable and you should look for the reason that caused that.

How Often Should I Change The Water To Remove Nitrates?

One large water change (50 percent or more) should be enough to dilute the nitrates. Large water changes are tricky because high nitrate levels are harmful to fish.

If your fish are already sick, they may not appreciate a large water change. In fact, large water changes can kill weak fish because of the stress they induce. 

If your fish are too weak and stressed to survive a significant water change, perform smaller water changes daily. 

You can also apply water conditioners. They will lower nitrate levels without harming the fish. 

But water conditioners are a short-term solution. Once your fish are strong enough, you should perform a large water change.

Some aquarists will move sick and stressed fish to a separate tank before giving the main tank a thorough cleaning. 

How Long After A Water Change Should I Test For Nitrates?

You can test for nitrates immediately after performing a water change. Ultimately, even if the first results are wrong, you can test the water again a few hours later.

However, it is common practice to wait an hour. The goal is to allow the new water to mix thoroughly with the old water. 

You may conclude that your tank has zero nitrates when you only tested the new water. The reverse is also possible. 

Old water may trick you into believing that your nitrate levels are still high. Wait for the new water to dilute the old water before performing a test.

It is essential to test the water source at least once when conducting a water change.

What Filters Remove Nitrates?

As mentioned earlier, nitrate can remain high after a water change because the water source itself contains nitrate.

If that is your case and you wish to fix that issue, there are a few options at your disposal:

1. Ultrafiltration

Ultrafiltration systems are fascinating because they mimic reverse osmosis in many ways. They remove pollutants by forcing water through a semi-permeable membrane using hydrostatic pressure.[6]

The filtration system removes everything from endotoxins and viruses to silt and proteins. 

However, ultrafiltration attracts a lot of debate because it isn’t necessarily an efficient nitrate removal method.

A paper by Brian Lee Hackman (University of Illinois) commended ultrafiltration systems for removing suspended solids and natural organic materials.[7]

However, Hackman admitted that ultrafiltration is not an effective pretreatment method for RO (Reverse Osmosis). 

But that doesn’t mean you should write ultrafiltration off completely. 

A study investigating various surface-modified membranes (the University of Osijek, Faculty of Food Technology) enhanced ultrafiltration’s efficiency as a nitrate-removal method by modifying three commercially available ultrafiltration membranes.[8]

In other words, an expert can probably build an ultrafiltration system for your house or aquarium that removes nitrates.

Although, as you may have guessed, customized filtration methods are expensive and rarely worth the trouble, especially when cheaper alternatives are so readily available. 

2. Nanofiltration

Like ultrafiltration, nanofiltration has a lot in common with reverse osmosis. Scientists innovated the filtration method in the 1980s to combat nitrates, viruses, and other micropollutants. 

Perfect for treating groundwater with high nitrate levels, nanofiltration can operate in low-pressure environments. 

A study from the Department of Engineering (Tarbiat Moallem University, Iran) and the Department of Environmental Engineering (University of Tehran, Iran), investigated the efficiency of nanofiltration as a nitrate-removal method.[9]

It found that the right membrane could remove as much as 95 percent of the nitrates in groundwater. 

Like ultrafiltration, an expert can build a customized nanofiltration system for your house or aquarium. 

You can also buy one of the many nanofiltration systems on the market. Some of those systems were designed with aquariums in mind.

However, you should look for a whole-house filtration system, especially if you rely on tap water during water changes. 

3. Carbon

A surprisingly common solution to nitrate contamination is a point-of-use filter. Point-of-use filters treat water at one tap.[10] However, the efficacy of a point-of-use filter depends on the filter media. 

Activated carbon filters do wonders for contaminants. People typically use them to remove strange flavors and bad smells from tap water. 

However, they can eliminate up to 80 chemicals and pollutants, including chlorine, chloride, phosphates, microplastics, and nitrates.[11]

Chemicals stick to the carbon when water flows through carbon filters. 

If nitrates are your only concern, experts from the Department of Environmental Engineering (University of Tehran) and the Department of Environmental Engineering, Science and Research (Islamic Azad University) investigated the effectiveness of activated carbon as a water treatment tool.[12]

The activated carbon, which they sourced from rice husks, was shown to lower the nitrate concentration in the water down to permissible levels.

It is worth noting that activated carbon is not the only absorbent on the market. Some studies have identified clay as an effective nitrate remover as well.[13] 

But conventional aquarists don’t have the time or patience to engineer a clay-based filtration system for their house or aquarium.

4. Distillation

Distillation technology works with carbon filters to remove impurities from the water. Distillation systems boil water until it evaporates. 

This is good because contaminants cannot evaporate or take on a gaseous form. Therefore, they will stay behind.

The evaporated water will continue to the carafe, where it will condense into liquid form. The carbon filter will eliminate any pollutants that remain. 

Besides removing nitrates and nitrites, distillation appeals to aquarists because it eliminates bacteria and heavy metals.[14]

Distillation won’t be your first choice because of the delays involved. It can take several hours to produce a gallon of distilled water. 

Then again, you can purify and store water in a container for future water changes. 

5. Reverse Osmosis

This filtration system works by forcing water through a semi-permeable membrane. Reverse osmosis is more effective than other filtration systems because their membranes have microscopic pores.

In other words, the membrane is fine enough to sieve ions, stopping nitrates and nitrites in their tracks. While the pure water continues to a storage tank, the contaminants will drain away.

The average RO system can remove 98 percent of all contaminants, including 92 percent of nitrates.[15]

Reverse Osmosis won’t remove all the nitrates. A test will reveal trace amounts of the substance, which is acceptable.

You don’t have to remove all the nitrates to protect your fish. The toxin is only dangerous in high concentrations.

You can attach RO filters to an aquarium. However, it makes more sense to install a whole-house RO system.

An image of a cleaning filter based on reverse osmosis (RO).

6. Ion Exchange

Ion Exchange uses cartridges with nitrate removal resins to purify the water.[16] You can buy nitrate-selective ion exchange resins or the standard variety.

Standard resins can remove other ions, including arsenate and sulfate.[17] The ion exchange process is nothing new.

It is typically used in water softeners, which works in your favor if the tap water in your region is extraordinarily hard.

Ion exchange nitrate filters have a limited lifespan. They can only purify a certain amount of water.

Eventually, the filtration process will exhaust the chloride ions. Once that happens, you must regenerate the filter media.

Although, many aquarists prefer to replace the cartridge. If you check the filter’s documentation, it will tell you the rated gallon capacity lifespan.

Test your tap water for nitrates. The concentration will show you the rate at which the media will deteriorate.

Naturally, media exposed to higher nitrate levels deteriorates at a faster rate. Make sure the filter’s capacity can accommodate the nitrate concentration in your area. 

You should also keep the sulfate levels in mind. An ion exchange resin will engage the nitrates and sulfates in a water supply with both substances. This is a problem because the resin will prioritize the sulfates.

If the water supply has more sulfates than nitrates, the filter will remove fewer nitrates. The filter’s efficiency as a nitrate remover will fall dramatically.

An ion exchange filtration system.

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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It is pretty common to measure high nitrate levels after a water change. The main question is how high the nitrates are, as fish can tolerate up to 20ppm.

Bear in mind that some water sources contain nitrates, so you might be adding them yourself when conducting a water change.

Therefore, besides measuring the aquarium’s chemistry, I suggest performing one test on the water added to the tank.

Ultimately, make sure you eliminate other common sources of nitrate, including decomposed organic matter, which is usually stuck inside the substrate.


  1. https://www.thesprucepets.com/water-changes-1381886
  2. https://etd.auburn.edu/bitstream/handle/10415/4974/Shamim%2520Naigaga%2520thesis.pdf?sequence=2
  3. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0023281
  4. https://doh.wa.gov/community-and-environment/drinking-water/water-system-assistance/group-b/general-information/nitrate
  5. https://link.springer.com/article/10.1186/2052-336X-11-35
  6. https://crystalquest.com/pages/what-is-ultrafiltration
  7. https://www.academia.edu/24027046/Membrane_Technologies_for_the_Removal_and_Nitrates_and_Atrazien_from_a_Surface_Water_Source
  9. http://www.ipcbee.com/vol17/44-L30009.pdf
  10. https://www.epa.gov/sites/default/files/2017-12/documents/lower-yakima-valley-groundwater-faq-october-2012.pdf
  11. https://tappwater.co/en/what-activated-carbon-filters-remove/
  12. https://www.researchgate.net/publication/303613211_Removal_of_Nitrate_from_Ground_Water_Using_Activated_Carbon_Prepared_from_Rice_Husk_and_Sludge_of_Paper_Industry_Wastewater_Treatment
  13. https://www.hindawi.com/journals/tswj/2019/9529618/
  14. https://waterfilterguru.com/how-to-remove-nitrates-from-well-water/
  15. https://www.freshwatersystems.com/blogs/blog/how-to-remove-nitrates-from-water
  16. https://www.uk-water-filters.co.uk/water_problem_nitrates.html
  17. https://pureaqua.com/nitrate-removal-from-water/