Nitrites (NO2) and nitrates (NO3) in a freshwater aquarium

As I am writing this post, I am measuring the nitrates level (NO3) in our freshwater aquarium as I do at least once a week. My level today seems to be somewhere between 10 and 20 ppm. As it has been as long as I can remember.

Now, especially if you are from North America (based on my experience on some forums I visit) you might think “How come he says as long as I can remember?” or “OMG, you must change your water!”. For some reason on some forums, without mentioning any names, changing the water once a week is an obsession for some and having a level of 20 ppm nitrates in your tank is somehow crazy. Well, I disagree.

As you know, nitrites turn into nitrates through the nitrogen cycle. This is good. A well functioning freshwater aquarium has a level of 0 ppm of nitrites in the water. If this goes up, time to change the water and start thinking of the reasons why nitrites levels are up (dead fish in the water, too much feeding, old filter system.. or something else). But nitrates is different. Actually, to have nitrates in the water is good because of the plants. The plants need nitrates (actually they need either ammonia or nitrates, but you don’t want to have ammonia in your water. At all).

In my freshwater aquarium, I have plenty of plants. These plants will use the nitrates in the water for their growth and turn the wasteful nitrates into proteins – and more beautiful plants. It’s really like a good cycle; I get lower levels of nitrates and at the same time greater plants. And this has been going on for as long as I’ve had my Juwel RIO 180 tank. And there is no reason to believe that this would change either, so I am really hoping to have low nitrates levels from now on.

To summarize, I have 0 ppm nitrites and 10-20 ppm of nitrates and with the amount of fish and plants I currently have, this is a stable ecosystem. If either the number of fish goes up, or the number of plants go down this stable ecosystem will change. Only you can know when and if you have a stable ecosystem. With this kind of stable ecosystem you don’t necessarily need to change the water that often.

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  1. All my fish died I took my tank do to only 25% of the water and changed filters and then added more water and when tested
    NO3 nitrate 80
    NO2 nitrate 1.0
    Hardness 75
    Alkalinity 80
    PH 7.2
    What should I do?

    • Autch, that doesn’t sound too good. But I am sure it can become better quite easily! Just be patient.

      The good news are that your pH level is good. And it’s also very good that you are equipped well enough to be able to measure your levels.

      But the most critical thing in the beginning is to focus on the your NO2 level. With that kind of NO2 level, no fish can survive. Please don’t add any fish until NO2 levels are 0.0. The state of your tank was basically such that all of the waste that your fish produced were not processed and transformed by the “good bacteria” to less toxic wastes. All of those toxins accumulated and made the water hazardous. Your fish died of ammonia poisoning. Basically your tank is not cycled (there’s no or not enough good bacteria in there to transform ammonia and NO2 into NO3, which is less hazardous). Now you need to take some time and make sure that you will get the good bacteria in there. The only way to do so is to let it grow.

      Now you need to let your tank mature (or be “cycled”). Add all the sand and plants you want to have in your tank, clean (not necessarily renew, if they are not old) the filters and let the tank mature; simply let the pump circle the water through the filters for at least a few week with the lights on 24h and do 25% water changes every few days.

      Measure your NO2 and NO3 levels daily. After a few days you should start seeing a drop in NO2 and a raise in NO3. This is normal in the beginning and indicates that the cycling process has started. This is good. After a week or so you should start seeing the NO3 leveling and slowly start to descend, which is normal. After a few days from this the NO2 levels should also start leveling and going down. In a healthy aquarium the NO2 level is 0.0 and the NO3 level is about 20-30. This is when the cycle is complete. This is important to remember and only now you can start adding fish. But I would still recommend to do this slowly as the ecosystem in the tank has to adapt to any new fish you add (and the bacteria has to be able to process their wastes as well).

      Even if your pet shop owner would recommend to add some “nitrite stabilizers” or “cycle accelerators”, I would not recommend those. You still need to change the water to get that poison out of the tank, so what those chemicals do is that they only postpone the water change. But when you are maturing your tank, this is not what you want.

      And as you probably know, your NO3 level is your indicator to when you latest have to change the water. When the level starts to be close to 50, it’s time to change the water. But in any case, your NO2 levels should always be 0.0. If not, change the water. If your NO2 level is going up, it means you don’t have enough good bacteria in the tank to cope with all your fish and/or you are feeding them too much. You need to let the aquarium “rest” (don’t give any food one day and feed less the next days).

      Good luck 🙂

  2. thanks, most people not even bother to use Nitrites (NO2) and nitrates (NO3)I have been lost for days…

    I do have a question as I am in china so getting the testing equipment is hard and translating is a (dog of the female type)
    I did manage to get a test for NH3N so I am told what is this and how do I use it

    once again love your post

    • Hi,

      Thank you for your comments!

      NH3N is ammonicial nitrogen and it measures the amount of ammonia in your tank. I am not a chemist, so you better read the details on Wikipedia (

      But essentially, you will always get ammonia into your tank from plant and fish waste. Ammonia is extreme dangerous. That is why you should have a fully functioning nitrogen cycle in your tank ( This chemical process turns ammonia into nitrites and nitrates.

      So essentially, you don’t really need to measure the ammonia level in your tank. It should always be 0 when your tank’s nitrification cycle works. Instead, you should measure that amount of nitrates and nitrites in your tank to verify that the nitrification cycle works, but also to measure the exact level of nitriates to see when you have to do a water change.

      Hope this helps 🙂

  3. Hi Kypeli

    I totally agree. I run an aquaponics system where I grow veggies and herbs. NO3 is a key ingredient in growing plants, in and out the water. Anyting under 50mg/l is safe.

    NO2 is very bad on the other hand. Anything above 1mg/l is bad.

    I would recommend a 25% water change if anything is above for mentioned readings.


  4. Hey, in my system no2 over 5 , no3 over 160 ,ph 7.2 , nh3-nh4 -1 , all my fish died. I change 75% water, and after 2 days is the same whiteout fish.

    • Those values are really bad. It sounds like your tank isn’t cycled (please see Or most likely you also overfeed your fish. If your NO2 is at 1ppm, your fish will die.

      What you need to do is take all your fish out or not put in new ones at least, change 50% of water, don’t feed the fish for a couple of days and get from your local aquarium store a solution that will start your aquarium (it’s a bacterial solution). After that, you have to feed your fish really really carefully and measure your no2 and no3 levels. Also ensure that your filters are clean, BUT don’t clean them too often because it will take out the good bacteria that lives there. Don’t clean them more than, say, every other week, tops. I clean mine once a month. It depends on how dirty they are, but they really shouldn’t be too dirty after one week, or otherwise you know that you overfeed your fish.

      When you no3 is over 30ppm, you have to make a 30% water change. But most importantly, don’t overfeed your fish. Feed only once a day. Depending on your number of fish as size of the tank, this of course may vary, but I suggest you read about it on the web.

  5. My ph is at 7.6
    High ph is at 7.4
    Ammonia is at 0.25
    nitrate no2 Is at 0ppm
    Nitrate no3 is at 5.00ppm
    Is that good for a tropical aquarium and would you say my tank is cycled and ready for fish

  6. Jaymee Mercado

    Need help,
    I just changed 50% water in my freshwater tank 2days ago.
    And now it say my Nitrate No2 is 5.0-10.0
    I didnt change my filter(I have 2) since I didnt want to loose all the good bacteria yet and since I just changed the filter 2weeks ago.
    Nitrate No3 is 20
    Nitrate No3 is 5.0-10.0
    Hardness 75
    Chlorine 0
    Alkalinity is 50
    Ph 7.0

    What should I do?

  7. Hey there,
    I recently got a tank on Sunday the 19th of Feb, I added water conditioner and also added filter boost every day until Sunday the 26th of Feb where I did a test. 0ppm of ammonia, Nitrite and Nitrates, which is fair enough because I hadn’t added any fish at that point. On Wednesday the 1st of March I did a 50% water change added water conditioner and filter boost, also 3 fish.
    Over the next week I was adding filter boost and testing the ammonia levels and using ammonia remover when needed and today Wednesday 8th of March is where I did another water change.
    This morning I checked the tank and found the goldfish, olive, who wasn’t eating as much as the others dead in the tank.
    I don’t want any of my fish to be ill especially if there is something I can do to fix it. When I tested my water after a 50% change yet again there was 0ppm ammonia, nitrite and nitrates. The pH being 6.0/6.4

    How do I get my tank to cycle? Is it cycled? Was it my fault my little olive died? Help

    • Hi!

      Your problem is probably not cycling the tank since you don’t have any ammonia, nitrate or nitrite. But a pH of 6.0 and 6.4 sound way too low. It should be as close to 7.0 as possible. Without knowing more, I would say the pH is the problem with the tank. Have you tested the pH of you tap water before adding any water conditioner?

      Olivia could have also died of natural causes, like stress from a new environment. It’s really difficult to say.

      But I would also advice you to not change 50% of the water at once, but more like 30%. Doing that every other week with very moderate feeding should be enough. But remember to test the water values to know what’s ok.

  8. Hi,
    I recently got a new tank and one of my beginner mistakes were adding new fish immediately after i bought it (with the shops water, wich was bad aswell) now , my fish all died (9 of them) in 2 weeks

    ph 7.1
    kh 3°d (the° needs a line under it)
    gh >16°d
    NO2 0
    NO3 25

    I do know that my ph is good but i’m a little worried about the rest.

    when some of my fish were still alive , my aquarium was developing this brown/yellow substance on the soil (white gravel). i got told these were biatoms that grew and it was fairly common to find in newly set up tanks. Then the other forum told me its part of the cycle? i got really confused.

    I hope you guys could help me out!
    Thanks in advance.

  9. hi im having trouble with my tank NO3 at very very high over 250ppm but NO2 is 0 and ph is low at around 6.5 i admit iv been slacking with water changes as iv been working alot recently. is this likely to be the only cause and can i get it normal without killing my fish its a 500L tank with 100L sump have 2 large birchirs fire eel and black ghost knife fish

  10. Great article! I’m fairly new to having a small fish tank. I used to have a beta that’s it. My beta died and I wanted more fish and interest. I have a Marineland Contour Glass 3 Gallon Aquarium Kit with Rail Light, Small Pump, and Filter. I purchased a rock that has a plant that grows on it, and a wood aquarium log. I cycled the tank and added two guppies, one nerite snail and one amano shrimp. Yes, they all are surviving in a 3 gallon tank! 😀 I read so many articles and if I had heeded those articles, I would only have one guppy or back to the beta. 😐 However, everything is awesome. 😛 The shrimp and snail keep everything clean from algae. The guppies are pretty and more active than my previous beta. At first, I was changing 20% of the water every 1-2 weeks, but my bad 😳 … because of time restraints I went 2-3 months without a water change, 😯 just added water when level dipped from evaporation. Testing the water, my levels were remaining at the safe levels. I just did a 25% water change because for the first time, the ammonia level went up from 0 to .5, which is still considered safe. Now the levels are Nitrate 10-20, Nitrite 0, water hardness is soft, Chlorine 0, Alkalinity 120-180, and pH 7.8-8.4. I am using filtered well water and have to deal with minerals in the water. I would like to add another guppy, but don’t want to push my luck.

  11. Hi, my freshwater aquarium is
    Ph 7
    Ammonia 0.25 ppm
    Nitrate no2- 0.10 ppm
    Nitrate no3- 0.5 ppm
    Is this safe? I think the ammonia and nitrates are a bit high due to over feeding. I am new to this but ive done some research. Am I right in thinking this could be why my ammonia and nitrates are am bit high? Thank you mel

    • Hi,

      I would not worry about nitrates (NO3) as it can safely up to 10ppm. Or that’s even preferred as your plants need NO3 and will absorb it naturally.

      But your ammonia and nitrite (NO2) should be at 0ppm. Because they are not, this would indicate to me that you are overfeeding your fish and/or your filtering system is not working properly, by it being either too dirty or bacteria have not yet cultivated the filtering system, which is necessary for the nitrogen cycle to work. So my advice is to feed your fish minimally, do a partial (no more than 30%) water change and keep on measuring until your ammonia and NO2 hits 0ppm. Until then, restrain from overfeeding the fish.

      Good luck!

  12. Hey there.
    For years I set my tanks up with a layer base of normal potting soil covered with fine silica sand. Then I plant up and leave to settle. In the shortest time I have a ‘jungle’ in my tank. Often I neither aerate nor filter and never change water. I top up with municipal tap water that I leave in the sun for 24hrs.
    I’ve just set up a 40litre tank for my kid and is much smaller than I’ve previously tried. I’m looking for an ‘Amazon stream’ tank for Tetras. I got lots of plants and drift wood in the tank. The water is ‘tea’ coloured and looks like a tanin rich stream. But I’m curious about the low concentration of Nitrate/Nitrite though the ratio of NO2@ 1mg/litre : NO3@ 10mg/litre seems good. With the high level of organic material in the plants, driftwood and potting soil shouldn’t the nitrate and nitrite concentration be higher?

    • Hi Dylan,
      I would say that your numbers seem good to me. If you have lots of live plants in the tank, they will absorb the nitrate which could explain the level of nitrate you see in the tank. Also on the other hand, to my knowledge driftwood and potting soil on their own do not contribute to higher NO2 and NO3 levels on a short term. The levels of NO2 and NO3 are much more affected by the number of fish in the tank, hence the amount of food you give them and how often you change the water (if ever, in your case). In general, I think it’s much more important to have a stable level of NO2 and NO3 which indicates a balance in the tank.

      So keep on measuring the levels and if your fish and plants are healthy and grow well, don’t worry 🙂 Good luck!

  13. so, I am new to this. Bought my fellow a tank for his birthday a few years ago – and it lay empty for 2yrs. so this birthday I filled the tank, put in gravel that was cleaned, some driftwood, a few mossballs and planted real plants. We let it sit to cycle for 3 weeks and the plants are growing beautifully. We then added fish – I now know that was probably too early – because the nitrates and nitrite levels went crazy. I have been doing daily partial changes of about 20% but they went so high that today I had to do 3 partial changes of 30% and still the nitrites are slightly high. this is using the 6 in 1 strips. the fish seem much happier though and are active – no red gills anymore (whew!)we have a 120ltr tank with a filter good to 150ltr. in the tank are 2 corys and 6 platys. at least one of the platys are pregnant. the levels say no3 is 10 now, no2 is about 5, gh kh an ph are all well within the normal and ci2 is between 0 and 0.8. my question is how much should I be feeding them? I have both flake and bloodworm. I have been feeding them a pinch of each 3 times a day. this was what one page recommended but now I have read on another page that is way too much.

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