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Aquarium Nitrates; Lowering Nitrate Levels, How to Control

By Carl Strohmeyer-PAMR 40+ years experience
Updated 9/6/23



Although much less toxic than ammonia and nitrite; Nitrate (NO3) as a nitrogen compound can also cause stress making a fish’s organs work harder to adjust to their environment, especially at levels higher than 100 ppm in many fish.
The increasing stress results in the loss of ability to fight diseases, the ability to heal itself, and the ability to reproduce.
It is essential for you, the aquarium (or pond) hobbyist, to maintain a proper environment for your aquatic companions. High nitrate levels are often a sign of poorly maintained aquariums and will cause problems in the long term.

How to Reduce Aquarium Nitrates Video

As a GENERALIZATION (which is the general consensus among experienced fish keeping sources, but also not totally agreed upon), I recommend maximum levels UNDER 40-50 ppm for FW (shooting for numbers under 10-30 for more optimum conditions for many inhabitants).
20 ppm or less for Saltwater fish, & under 5 ppm or less for reef aquariums. For planted freshwater aquariums, about 15 ppm is suggested (or even higher, as too low in planted aquariums can be a problem).

It is also noteworthy that very low nitrate levels in an established aquarium does not necessarily mean that the aquarium is not cycled.
As often a low bio load and/or aspects of natural nitrate reduction such as plants (or Pothos), refugiums, anaerobic de-nitrification filters, etc. along with nitrate lowering methods such as Algone, NPX Bioplastics, Protein Skimmers, etc. can IN FACT keep nitrates at near ZERO numbers. I've seen this 100s of times in my career with my client's aquariums.

If levels exceed these generalized numbers for a LONG period, NOT just a day or even a week, coupled with other water parameter factors such as poor Redox balance, over crowding, poor feeding, and more, this can and will affect long term fish health.
Further Reference: Aquarium Redox

Aquarium Nitrate Control; Test KitLong term high nitrates are potentially dangerous due to the effects on the water chemistry and on a healthy environment for your fish while nitrates are accumulating.
The higher the nitrate levels, the higher and more severe the consequences due to the stress on your fish and the favorable conditions for a serious algae outbreak.

Nitrate levels around 5 ppm or less are found in nature which provides an almost nitrate free environment (although a Nitrate level of around 15 ppm (or even higher depending upon fert dosing) is usually best for planted freshwater aquariums as noted earlier).
The higher the the long term nitrate concentration the more stress for the fish. More severe stress is reached at levels exceeding 100 ppm.
Additional stress MAY be added due to an accumulation of life forms such as detritus worms feeding on decomposing waste, and the consequently higher biomass (organisms living in the aquarium) leads to an increasing demand of oxygen.
Reference: Aquarium Planaria? Actually Detritus Worms

I will also add that although nitrates are not dangerous in the short term unlike ammonia or nitrites; in established tanks I usually test this parameter more often as this is a good indicator of how well I am doing in my tank cleanings.
Example; if enough water is being changed and with the correct frequency, as well as to whether additional nitrate removing products or procedures need to be employed. Tests of KH & GH are also useful in indicating an established tanks health.
Reference: Aquarium Cleaning; Reasons, Frequency, more

One more point about nitrate; I have tested the water on under sized aquariums/bowls containing otherwise healthy goldfish.
The nitrates would often exceed 200 ppm! These goldfish (although they appeared healthy), rarely lived more than 3-5 years as compared to the 12 + years of the goldfish I have kept for clients in pond and larger aquariums.
While not fish, University studies in Cattle show nitrate levels in water over 221 ppm to be harmful or even fatal, so I sure would NOT want my fish kept at nitrate levels approaching this number.

This said, if your nitrates are not always increasing (which constantly increasing nitrates can be an indicator of a high bio load or poor fish keeping practices), I would not go overboard in the average aquarium with a stable nitrate level of 40 ppm or less by spending lots of $$ on nitrate control products or making constant large stressful water changes (small scheduled changes are part of normal fish keeping as per my Aquarium Cleaning Article, but the need to make 50-100% water changes makes me question bio load, filtration feeding, etc))
Reference: Aquarium Cleaning



Nitrates (NO3) are compounds composed of a nitrogen and three oxygen atoms and are often the final stage in the nitrogen cycle of fresh and saltwater aquariums if there are not nitrate removing plants, algae, or nitrate reducing anaerobic bacteria present.
Nitrates are the conjugate base (chemical substance that releases a proton in the backward chemical reaction) of nitric acid (HNO3), consisting of one central nitrogen atom surrounded by three identical oxygen atoms.
The presence or production of large amounts nitrates can result in the presence of Nitric acid according to the Brønsted-Lowry theory of acids and bases which will in turn affect an aquariums pH and KH (which can result in dangerous pH swings).
This will also have an affect on your aquariums Redox balance and likely result in higher oxidative stress on your aquarium's inhabitants.

For more about the nitrogen cycle, please reference:
*Aquarium Nitrogen Cycle

Aquarium Nitrates, Bio LoadA high Bio-Load that often produces large amounts of organic mulm and decomposition in an aquarium (or pond) gravel or in filters is often a common cause of persistent nitrate problems.
Another clue to this is a pH that tends to drop quickly, often even with buffers added (assuming a higher new water pH); the breakdown of organic mulm or similar will lower pH while increasing nitrates.
Pockets of decomposing organics are often found in areas of deep fine sand, under rocks or other décor, or in large filters (especially canister filters).


High nitrate levels which many sources based in human studies place as low as 30- 45 ppm of nitrate as harmful. The EPA recommend levels under 10 ppm in drinking water, although often this can higher especially in well water.
As for fish, high nitrate levels can cause respiration problems in fish, lower or eliminate the ability to breed, resist disease, and lower activity of aquarium inhabitants.

My own experience and test have shown immediate problems with nitrate as low as 20 ppm with cephlapods in marine aquariums.
However I have kept many general fish from goldfish to cichlids at stable numbers as high as 50 ppm without any noteworthy documented long term problems. I have however noted/documented some long term issues with constant numbers above 50, especially above 100 ppm and higher.
Because of this, as noted earlier, unless your nitrates are showing constant upward trends after every water change, which might indicate a higher bio load than your aquarium system is capable of, do not get too overly concerned with numbers under these.
Further Reading: Bio Load in Aquarium or Pond

Studies show that the toxicity of nitrate is due to nitrite being an intermediate. Nitrites oxidize the iron atoms in hemoglobin from ferrous iron (2+) to ferric iron (3+), rendering it unable to carry oxygen.
This process can lead to generalized lack of oxygen in organ tissue and a dangerous blood condition called methemoglobinemia.
What is noteworthy is that NITRITE also needs to be present, this is why current evidence does not support those in some fish keeping articles that state that anything above 20 ppm nitrate is toxic, as this these assumptions were likely based on nitrite also being present since current scientific evidence shows that nitrates by itself cannot cause these issues at levels at say 30 ppm.
The evidence also suggests that since nitrites were also present, likely such an aquarium was not fully cycled.

In human studies high nitrate levels have been shown to dangerously lower blood pressure by causing the muscles that control the size of blood vessels to relax, this can be dangerous to fish too causing circulatory problems which can again result in poor disease resistance.
High Nitrate levels in aquariums will also result in high algae growths and in marine aquariums is toxic at even low levels to Cephalopods such as Octopus and to corals, although many freshwater fish can tolerate high levels for quite some time based on my experience.

Another danger is in the bloodstream, nitrates can be converted biologically to nitrites, leading to "Brown Blood Disease".

For more on the potential dangers of Nitrates:
*Wikipedia; Nitrate
*Nitrates In Drinking Water
*Wikipedia; Nitrate

Temporary Relief of Nitrate Poisoning:
First, please note that high nitrates are NOT EVEN CLOSE to the danger of high ammonia or nitrites for fish, so if these are the problem, high ammonia and nitrites should be addressed first.
Some have stated that moving from high nitrates to low nitrates or vice versa can also cause nitrate shock similar to pH shock, however this is anecdotal and my tests as well as research have yet to verify such claims (for one nitrates are NOT algorithmic like pH is).

Methylene Blue for Aquarium, aid fish nitrate poisoning*Medicated Baths using Methylene Blue can increase oxygen in the blood and quickly remedy ammonia, nitrite, and especially nitrate poisoning (in this order of effectiveness too with ammonia poisoning the least effective and nitrates the most effective).
See Reference: Performing Medicated Fish Baths, Dips, similar

Product Resource: AAP/Kordon Methylene Blu

Spirulina Algae aid for aquarium fish nitrate poisoning*Spirulina Algae or Chlorophyll Remedy; these build the immune system and increase blood oxygen levels in fish that have suffered from nitrite or nitrate poisoning or oxygen deprivation. Spirulina is the better choice of the two, being much better at increasing immune function.

The most simple and effective way to administer Spirulina is via a high Spirulina based food such as AAP Spirulina 20. Feeding Spirulina based foods although not a replacement for lowering your aquarium or ponds nitrate levels can be a reasonable albeit partial solution to chronic nitrate problems for many fish (not reef inhabitants such as Corals, Cephalopods, etc.).

If you have access to food grade Spirulina or Chlorophyll, these can also be used as a bath:
Pre-mix 1 ounce of spirulina or chlorophyll per gallon of aquarium water (I suggest first liquifying the powder with sterile water to a make a liquid ounce); allow fish to soak for 15-30 minutes; perform this once or twice a daily. Use a fresh bath for each bath using your display tank water.
This can be performed in 1/2 ounces with 1/2 gallon too for smaller fish.

Please read this article for more about Spirulina Algae:
Spirulina Algae

Where to Purchase, Product > *Spirulina 20 Fish Food Flake

*Most importantly, follow the steps below to lower your aquarium or pond nitrates in the first place and often any possible nitrate poisoning issues will go away too.


Here are a few basics for removal/ prevention of nitrates (I will add to this list over time too)

Further Resources:

Please reference this other Aquarium answers article for more other filter media types as well (including products for phosphate removal), a MUST READ:
Aquarium Filter Media; Types, Capacities and more

Please reference this article for more about the production of Hydrogen Sulfides during the process of De-Nitrification:
Hydrogen Sulfide production in anaerobic De-Nitrification for Aquarium/Pond Nitrate Removal

For a map of Nitrates in World Rivers, please click below
Nitrate Levels in Major World Rivers

For more referenced aquarium information pertaining to:
*Freshwater Aquarium Care, Basics to Advanced
*Saltwater Aquarium Care, Basics to Advanced

Other Recommended Reference & Product Sites/Videos

AAP Spectrogram
One of the most effective medications for the treatment of Columnaris in an aquarium when used as part of the four step program of Columnaris treatment.
A more synergistic combination than purchasing Kanamycin & Nitrofurazone separately.


AAP Yellow Powder; The Premium Nitrofuarazone combination; for Columnaris and Aeromonas next to Spectrogram (as Spectrogram is often not available), the best treatment for Columnaris, despite bad search results recommending Terramycin (oxytetracycline) and other inferior treatments for Columnaris

Columnaris in Fish Video
YouTube; How to: 4 Steps Columnaris Treatment Fish Bacterial Infection

This video goes over the basics of the full four step plan of properly treating Columnaris in aquarium fish and is a compliment to a FULL reading of this article.

The article below is a MUST READ for anyone interested in moving from basic aquarium keeping to more advanced aquarium keeping, including better Redox Balance:
Aquarium or Pond UV Sterilization
Ultraviolet Sterilization, Advanced Aquarium Keeping

Pond Care Information

Premium Tropic Marin Pro Reef Sea Salt from Germany

Premium Tropic Marin Pro Reef Sea Salt from Germany

There is simply NO BETTER Reef Sea Salt (marine fish too)

UV Replacement Bulbs, LampsUV Replacement Lamps/Bulbs; Aquarium or Pond
For TRUE High Output, Hot Cathode, Low Pressure UVC Germicidal Bulbs, for aquarium or pond

*Sponge Filtration; Complete Information about the use of Sponge Filters in Aquarium or Pond

Fish as Pets with articles & commentary of Interest to the Aquarium Hobby

Aquarium Power Head PumpSunSun JP-023 Aquarium Power Head Water Pumps

Superior performance and value when compared to many more well known brands such as Hagen or Marineland


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In Chronological order of writing with the newest at the top

  1. How to Treat Sick Fish
  2. Whirling Disease in Fish
  3. Reef Aquarium Chemistry Maintenance
  4. Use of RO, DI, Softwater in Aquariums
  5. Lighting Theory of a Planted Aquarium- RQE, PFY, PAS, & PUR
  6. Aquarium or Pond Bio Load
  7. Tuberculosis in Fish
  8. PUR vs PAR in Aquarium Lighting
  9. Head Pressure in Aquarium and Pond Water Pumps
  10. Fin/Tail Rot For Betta & ALL Fish
  11. Angelfish Virus/Aids
  12. Activated Carbon
  13. Fish Baths/Dips as an aid to treatment
  14. Streptococcus gram positive bacterium in aquariums, Eye Infections
  15. Hydrogen Sulfide
    production in anaerobic De-Nitrification for Aquarium/Ponds
  16. Fish Shipping
  17. Aquarium Size, Fish Stunting
  18. Aquarium Algae,
    BBA & Brown Algae in particular
  19. Aquarium Salt (Sodium chloride) in Freshwater Aquariums
  20. Betta Habitat; Wild Bettas to Domestic Betta environment parameters
  21. HITH; Hole in the Head Disease
  22. Aquarium Protein Skimmers, Ozonizers
  23. Power Head/ Water Pump Review
  24. Molly Disease/ Mollies in an Aquarium
  25. Basic Fish Anatomy, Fin Identification
  26. Aquarium Moving/ Power Failures
  27. Octopus as Aquarium Pets
  28. Aquarium Nitrates
  29. Ichthyophonus protists, fungus in fish
  30. Aquarium and Pond Filter Media
    Types; Mechanical, Bio, Chemical
  31. Aquarium Water Conditioners (also Pond)
  32. Fish Parasites; Trematodes & Monogeneans; Annelids and Nematodes;
    Flukes, internal worms, Detritus Worms (often confused with Planaria), Micro Worms
  33. Aquarium Silicone Application;
    DIY Aquarium Repair & Glass thickness
  34. Pond Veggie Filters; DIY Bog Filter
  35. The difference between Plaster of Paris and Aquarium Products such a Wonder Shells:
    Identification, prevention & Treatment
  37. AQUARIUM TEST KITS; Use & Importance
  38. SEXING FISH; Basics
  39. Chocolate Chip, Knobby and Fromia Starfish
  40. Freshwater Velvet & Costia
  41. Usnic Acid as a Fish Remedy
  42. Aquarium Heaters; Types, information
  43. The Lateral Line in Fish, Lateral Line Disease
    or Head and Lateral Line Erosion (HLLE)
  44. Tap Water use in Aquarium; Chloramines, Chlorine
  45. Can Black Ghost Knife fish give an electric shock?
  46. Bio Wheel Review; Do Bio-Wheels really work?
  47. How do Fish Drink?
    Use of RO Water
  48. Cyclops, and Predatory Damselfly larvae
  49. Betta with Dropsy;
    Treatment and Prevention of DROPSY in all fish
  50. pH and KH problems in African Cichlid Aquarium
  51. Aquarium Gravel, which size?
  52. Blue green algae, Cyanobacteria in Ponds/Aquariums

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*Sponge Filters that far outperform all other brands or DIY:
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Fish Food Information:

Aquarium & Pond UV Sterilizer Use Articles
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