Safe Nitrate Levels

5.4. Safe Nitrate Levels


All discussions of nitrate must be predicated with a caution. The 10 ppm of nitrate of the scientists, (the Hach test kit and the Lamotte test kit) is a 44 ppm test level of nitrate via the “normal” aquarium test kits (API aquarium test kit, the Seachem Multitest kit, the Sera Nitrate test, the Salifert test kit and the API test strips).

One Way to Look at Nitrate

Aquarium nitrate levels are a very gray area that is best looked at with a human analogy. Let us equate 40 ppm of nitrate per the API test kit (10 ppm per the scientists’ test kits) health-wise with one beer per day in humans.

One beer a day, every day, will create microscopic changes in the liver of a human, just like 40 ppm of nitrate causes microscopic changes in the liver of a fish. Does one beer a day hurt a human? Depends on who you talk to. Most research says one beer a day is harmless to humans. Also most research says 40 ppm of nitrate is harmless to adult tropical fish, fish fry, shrimp and axotyl.

Aulonocara Sp. Lwanda – Aulonocara Red Top
Aulonocara Sp. Lwanda – Aulonocara Red Top

Two beers a day, every day, might or might not create some slight health problems. In the same way 80 (40 x 2) ppm of nitrate in an aquarium might or might not create some slight health problems in an adult fish. It once again just depends on who you talk to. Researchers have found 80 ppm can damage shrimp, axotyl and some sensitive fish fry.

Four beers a night, every day, will probably create some health problems and give a human a shorter life span by 2 to 5%. The same is probably true with 160 (40 x 4) ppm of nitrate with adult fish. Is the risk acceptable? Depends on who you talk to.

Eight beers a night or 320 (40 x 8) ppm of nitrate will definitely have serious consequences. But some people live to 100 with eight beers a night.

Symphysodon aequifasciatus – Discus
Symphysodon aequifasciatus – Discus

Sixteen beers will kill some people. 640 (40 x 16) ppm of nitrate will kill some fish. So the analogy holds quite well. Note that both alcohol and nitrate are detoxified by enzymes in the liver of higher-order animals. Thus there are good reasons that the analogy holds well.

There is no right answer here. It just all depends on the risk level each hobbyist is comfortable with. Most hobbyists are comfortable with anything from 40 to 80 ppm of nitrate (i.e. one to two beers).

Note that nitrate is simply not very toxic to ADULT fish. António et. al. 2017 found no long-term effects from a level of 440 ppm. Monsees et. al. 2016 found the lower long-term damage level for adult cichlids to be 2,220 ppm. Science Direct and Semantics Scholar have several papers available all of which say a level of 440 ppm does no long-term damage to adult fish. A recent study exposed young salmon to 440 ppm nitrate for 8 months and they grew just as well as salmon kept at 1/10th that level.

Mesoheros gephyrus
Mesoheros gephyrus

Summary of the Research

Nitrate is another “toxin” frequently mentioned in the keeping of tropical fish. The toxic level of nitrate is quite high.

The following are the limits on nitrate:

  • Chronic: 440 to 1200 ppm depending on species
  • Acute: 880 to 4,400 ppm depending on species

Toxicity depends on the species, with pond fish generally being better able to handle high levels than fish from mountain streams. Because of inadvertent selective breeding, common goldfish can withstand very high levels of nitrate (>4,400).

Clown loach
Clown loach

The level of nitrate one wants to hold determines the frequency of the water changes. The lower the nitrate level one tries to hold the more one has to change the water. And the more one has to change the water the more likely it is that one will give up the hobby in a short period of time. Trying to hold a level of 20 in an aquarium will require four times the number of water changes than trying to hold a level of 80 in your water. Note this assumes there is no nitrate in the tap water being used.

With the popular API test kit for nitrate, it is very difficult to find any difference between 40 and 80 ppm nitrate. Aiming towards a “orange-red” but not “red red” on the API test is a very reasonable course for most hobbyists. Note that the API test is inaccurate if the darkness of the tube is used to gauge the nitrate. One has to use the chroma of the tube, namely how orange is the red.

The nitrate test itself is very prone to aging and inaccuracies as well. If the bottles have been open for more than six months at room temperature, the test can give erroneous readings. And the shaking directions need to be very closely followed. If they are not followed one will get erroneous readings.

Andinoacara pulcher – Electric Blue Acara
Andinoacara pulcher – Electric Blue Acara

Adults of higher-order animals like fish (and humans) have evolved metabolic pathways which neutralize nitrate quite efficiently. The only health issue in humans with levels of nitrate above 220 ppm in drinking water is blue baby syndrome. Newborn babies can’t handle the nitrates, but adults are fine with it.

Note that a scientist’s 1 mg/L NO₃-N translates to a hobbyist’s 4.4 ppm NO₃, thus the 44 and 440 figures.

Chaetostoma milesi – Rubber Lip Pleco
Chaetostoma milesi – Rubber Lip Pleco

The Profit Motive

There are a host of charts and websites which say one should do a 50% water change if you have 20 ppm of nitrate (i.e. a level of 10 ppm or less must be maintained) in order to have healthy fish. This typically translates into a 50% water change once a week. Some simple calculations say that the market for aquarium water conditioners is about $150 million if hobbyists do 50% water changes once a week.

If hobbyists only change the water when the nitrate level goes to 80 ppm (i.e. a level of 40 should be maintained, a far more realistic number) then the yearly market for conditioners is only $37 million. So that basically is 113 million reasons for the charts and the recommendations. Isn’t the profit motive great?

Vieja Bifasciatus
Vieja Bifasciatus


Whenever the topic of the effects of high nitrates comes up there are a whole host of people who come out of the woodwork saying this and that. Comments like “nitrates over 20 ppm are sure to create hole-in-the-head in his cichlids”. This sort of “statement of fact” is nothing more than unsupported speculation that has somehow through the magic that is the internet becomes “established fact”.

An extensive search of the research literature about hole-in-the-head has revealed absolutely no research which documents that nitrates are a factor in hole-in-the-head. The author had an Oscar some forty years ago when “old water was good water” and we didn’t do water changes. The nitrates must have been through the roof. The Oscar never got hole in the head. But the aquarium had several huge filters on it and the water was crystal clear.

There are a whole host of such unsupported claims. “Your fish’s color won’t be good if nitrates are high“, “Discus can’t spawn if nitrates are over 20“, “blackwater fish are super sensitive to nitrates“, “I’ve bred Africans and I can tell you my fish are healthier when I keep nitrates lower than 20 ppm“, etc. etc. All this is simply unsupported speculation.

Congo Tetra
Congo Tetra

I like the Facebook comment below:

Keeping your nitrates below 40 is a fools errand and completely unnecessary. People telling you to keep your nitrate levels consistently below that are simply parroting what they’ve seen other tell them, and have never actually experimented and tried things out for themselves. My tank started thriving more than ever when I stopped worrying about that kind of crap and messing with my tanks weekly. Switched to monthly water changes, and just looking to keep Nitrates < ~80-100ppm, everyone seems much happier. Eggs everywhere, Corys breeding, I even have zebra danios successfully breeding with fry surviving my angelfish. People try to do too much in this hobby.

Brett James Greig

Couldn’t say it any better if I tried.

Herichthys Carpintis, Green Texas Cichlid
Herichthys Carpintis, Green Texas Cichlid

Low Nitrates

There are constant questions on social media along the lines of “My tank has been running for three months and I have no nitrates. What is wrong?”

There are five conditions that can cause this:

  1. A lightly stocked aquarium being lightly fed
  2. Plants in the aquarium absorbing the nitrate
  3. Frequent water changes removing the nitrate
  4. Not shaking the API nitrate test tubes enough (a very common mistake!)
  5. Feeding food with less than 40% protein in it. This gives high rates of what is called “assimilatory denitrification”. Basically, the nitrate gets tied up in the brown gunk in the aquarium.

Obviously, all five things can go on at once in an aquarium. Note that “anaerobic denitrification” by something like a deep sand bed is not included. Deep sand beds do NOT do denitrification.

albino Cynotilapia zebroides Cobue
albino Cynotilapia zebroides Cobue

Calculating Nitrate Level

Hobbyists always want to know what their nitrate levels “should be” And actual exchange on this website is helpful:


How to calculate the nitrate ppm produced in a week in an overstocked 100gal aquarium being fed 7g of 40% protein food each day (49g total food per week), assuming the food is 100% consumed by fish? In short: how to convert grams of food to ppm nitrate? Thanks!


In reply to John ….. Let’s see it this very old brain can do this. 7×7=49 grams dry food per week, 40% protein, protein is generally 15% elemental nitrogen (with considerable variation). So we are adding 49×0.4×0.15= 2.94 grams elemental nitrogen per week. Nitrate is 4.4 times elemental nitrogen 2.94×4.4=12.9 grams nitrate per week. A single ppm in a 100 gallon tank is 0.36 grams. 12.9 divided by 0.36 = 36 ppm per week nitrate added. So for a 100 gallon tank one can just multiply the weekly weight of dry commercial food by 0.73 (36/49) to get the nitrate loading in ppm.
Note this assumes adult fish which are not adding weight to their bodies. If they are juveniles this number can easily be cut in half by the protein in the food ending up as muscle protein in the fish.

Sciaenochromis fryeri OB – Blueberry
Sciaenochromis fryeri OB – Blueberry

Further Data

There are some aquarium hobbyists who are interested in delving deep into the science and the calculations behind all aspects of the hobby. For those who are so inclined, the following is pertinent:

5.4.1.Nitrate in More Depth

Startpage Aquariumscience

Source: – David Bogert

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