How To Lower Nitrite In A Fish Tank – Freshwater & Saltwater

    How to lower nitrite in a fish tank

    Nitrite intoxication is not as common a subject as ammonia levels are. Or even nitrate levels. But it is just as dangerous to freshwater tank health. Knowing how to lower nitrite in a fish tank is an essential skill every responsible fishkeeper should know. But what exactly is nitrite?

    Nitrite is created when ammonia is only partially detoxified. It is still dangerous to fish and invertebrate health. And needs to be removed through water changes and beneficial bacterial action.

    Nitrite intoxication is less of an issue in marine systems, as you’ll come to discover. But if you detect it in your reef tank, nitrite still indicates problems in your aquarium nitrogen cycle!

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    What Are Nitrites In A Fish Tank?

    Ember tetra (Hyphessobrycon amandae)

    Nitrite is a chemical compound (NO2-). It is the first compound created by beneficial bacteria living in nature and inside of your fish tank.

    These bacteria are critical for the survival of healthy fish. Because without them, normal animal waste production would cause your fish to choke on their own pollution. Fish don’t create nitrite, however. So where does it even come from?

    Nitrite Levels And The Aquarium Nitrogen Cycle

    The aquarium nitrogen cycle works when ammonia (NH3)+ is released into the water. Fish excrete it as excess waste directly. Ammonia also comes from the decay of organic matter in the tank, especially protein. That’s why you often see ammonia spikes happen after a fish dies.

    This ammonia is highly toxic to fish and invertebrates. Fortunately, it is a source of food for nitrifying bacteria. Specifically, one group in the genus Nitrosomonas and Nitrosococcus. They eat ammonia, excreting it as nitrite (NO2-).

    Nitrite levels then start to rise. Nitrite is less toxic than ammonia is to fish. But it is still a dangerous pollutant. We’d have problems now, if not for the second set of beneficial bacteria in the genera Nitrobacter and Nitrospira. These nitrifying bacteria convert nitrite into nitrate (NO3-).

    Nitrate is far less dangerous than either nitrite or ammonia. However, it will become toxic if allowed to reach concentrations that are very high (10-40 parts per million, depending on the species).

    What Happens To Nitrate In Fish Tanks?

    Nitrate levels continue to rise because there are very few beneficial bacteria that feed on it. These are called de-nitrifying bacteria and they can be found in fish tanks. But only in small numbers because they are anaerobic organisms.

    As anaerobes, they find oxygen poisonous! In nature, they live by the trillions in mud, under rocks, in stagnant water, and in other low oxygen zones. They eat nitrate and convert it to nitrogen gas (N2). Thus completing the nitrogen cycle.

    Healthy fish tanks are oxygen-rich environments. So there will never be enough beneficial bacteria to break down nitrate. You need to step in and remove it with water changes. Or have enough live plants that they keep nitrate levels down by using it as fertilizer.

    How To Lower Nitrite In A Freshwater Fish Tank

    Nitrite is as dangerous as ammonia to freshwater tank inhabitants. There are four main ways of bringing nitrite levels down to a safe level.

    Do Partial Water Changes

    If the nitrite level is already in the danger zone, then perform an immediate large water change. Start with 25-50%, depending on what your nitrite test strips reveal. Remove tank water and replace it with fresh water. Always use a water conditioner like Aqueon Dechlorinator to remove chlorine and chloramine from your tap water.

    After you’ve refilled your fish tank, retest the tank for nitrite levels. If they remain high, wait a few hours and then perform a second water change.

    I would also test your tap water for nitrites. It’s not common but that could be a nitrite source, especially if you don’t have municipal tap water.

    Make sure you plunge the gravel vacuum deep into the substrate in the process. Pick up any fish waste, leftover food, and other sources of ammonia. Also, look behind decorations and aquarium technology for dead fish or invertebrates.

    Slow-moving or sessile (non-moving) invertebrates like corals, clams, and snails are the trickiest. Sometimes it’s hard to tell if they are alive or dead.

    One simple test is to give the animal a quick sniff. After just a few hours, a dead invertebrate starts to smell pretty foul. They decay fast at warm temperatures, as anyone who cooks seafood knows.

    Even if the snail or clam has closed off its shell, it will be stinky when dead. A live invertebrate, on the other hand, will smell fresh or like nothing at all.

    Add Beneficial Bottled Bacteria

    I recommend adding a booster of nitrifying bottled bacteria right after you perform a large water change. Extra beneficial bacteria never hurts.

    There may be a hidden dead fish or some other source of protein that’s leaking ammonia into your aquarium water. Your current population can’t handle all of it.

    They will reproduce more often to take advantage of the food surplus. But you don’t have to wait for nature to take its course. Especially if nitrite intoxication is starting to occur in your fish.

    Fortunately, booster shots of nitrifying bacteria are easy to find online! Simply follow the dosing instructions, wait a couple of days, and then re-test your tank for nitrite spikes.

    Use A Chemical Detoxifying Agent

    There are also chemical detoxifiers that target ammonia, nitrite, and nitrate. I recommend these if levels are out-of-control high. To the point where your fish are in danger of nitrite intoxication. But you don’t have time for water changes or to allow a new batch of nitrifying bacteria to settle in.

    The reason why these should not be your go-to solution is that they will disrupt your nitrogen cycle. If you eliminate ammonia, nitrite, and nitrate in your tank, what bacteria you do have run out of food.

    Your fish will create more ammonia but it will take a few days for nitrite and nitrate to re-establish themselves. The risk is small but I never like destabilizing my beneficial bacteria, if I can help it.

    That said, chemical agents are an excellent emergency solution for a sudden nitrite spike!

    Add Aquarium Salt

    Aquarium salt counteracts the ability of nitrite to poison fish. Obviously, you don’t need to add any to a saltwater fish tank. The natural water parameters are already salty enough.

    But adding one tablespoon per 5 gallons to a freshwater tank will ease stress in your fish. This level won’t cause stress to salt-sensitive species like tetras, gouramis, or betta fish.

    But don’t add much more than this unless you are sure your fish can handle it. Many freshwater fish find too much aquarium salt stressful beyond a brief dip for diseases.

    If your fish are salt-lovers (livebearers, African cichlids, etc), add one tablespoon per gallon. Maintain salt levels at this concentration until you can get nitrite levels back to 0 ppm.

    Besides countering nitrite poisoning, you can also use salt as a tonic for ich, velvet, anchor worms, and other diseases. Aquarium salt is a great medication that every freshwater aquarist should have on hand!

    What Is An Acceptable Nitrite Level For A Freshwater Aquarium?

    Nitrite in aquarium acceptable level
    Bleeding heart tetras (Hyphessobrycon socolofi)

    For freshwater tanks, nitrite levels should remain as close to 0 parts per million as possible. There is no safe level of nitrites in a freshwater aquarium. Even 1 ppm will cause stress to your fish.

    Ammonia and nitrite should be treated equally when it comes to how toxic they are. It’s well established that ammonia is very dangerous to fish and invertebrate life. But the exact toxicity is pretty variable.

    Water temperature, pH, and salinity all affect how your fish respond to ammonia and nitrite. So if your test kit shows low levels of either nitrogenous waste product you need to respond ASAP.

    How To Lower Nitrite In A Saltwater Fish Tank

    Actually, you won’t ever need to lower nitrite levels in a saltwater tank. Nitrite poisoning occurs when chloride ions are replaced with nitrite. That won’t happen unless nitrite levels are in the hundreds to thousands of parts per million.

    A few sensitive species of marine invertebrates show toxicity at around 30 to 60 ppm. But that’s far more resilient than any freshwater fish or invertebrate. Not to mention far above what any nitrite test kit will be able to display.

    If you do see high nitrites then you also have either high ammonia levels or high nitrate levels. Fortunately, both are easy to manage.

    You lower ammonia and nitrate levels the same way you do nitrite: water changes and adding extra beneficial bacteria to the system. All three nitrogenous waste products are flushed from the aquarium system this way.

    What Is An Acceptable Nitrite Level For Saltwater Tanks?

    Here is where things get interesting. Believe it or not, nitrite poisoning in salt water fish almost never occurs. Marine fish tanks have the same nitrogen cycle. And ammonia is converted to nitrite and then to nitrate.

    But nitrite poisoning is almost impossible because the normal uptake mechanisms are disrupted in saltwater systems. Let me explain.

    Nitrite poisoning occurs when the NO2- ion competes with chloride ions (Cl-) for the uptake proteins in the gills and blood. These proteins have an even higher attraction to nitrite. Nitrite then displaces CL- causing fish to suffer chloride depletion.

    Since chloride ions aren’t found at high concentrations in freshwater aquarium water, nitrite poisoning is dangerous. But that’s not the case for saltwater environments.

    Here, chloride ions exist at concentrations of around 19,000-30,000 parts per million. Nitrate levels would have to be impossibly high for the compound to out-compete chloride ions. Or put another way, the lower the salt concentration is, the more dangerous nitrite levels become.

    Do I Need To Lower Nitrite Levels In A Saltwater Tank?

    The simple truth is that you don’t need to lower nitrite levels in saltwater tanks. Nitrite poisoning can’t occur because chloride ions can’t be outcompeted in salt water.

    That said, you’ll find loads of sources claiming the opposite: that nitrite poisoning is dangerous to marine life. And you need to maintain 0 ppm at all times.

    There are two reasons why this outmoded idea keeps being repeated. The first is because much of what is true for freshwater tanks is also true for marine tanks.

    High ammonia levels are still dangerous to saltwater fish and corals. And nitrate levels should still be watched – but the compound is not as dangerous as ammonia. So it made sense to treat nitrite levels the same way.

    And the second reason is that there is no disadvantage to lowering nitrite levels. Doing water changes is a good thing, regardless. Plus it directly treats the actual causes of fish stress.

    Nitrite is an intermediate step in the nitrogen cycle. Therefore, high nitrite levels are a sign that nitrate or ammonia levels are also high. There is almost no situation where you will have high nitrite by itself.

    So if you have a saltwater tank, you should treat a nitrite spike as a sign. They indicate that you need to test your ammonia and nitrate levels. Because one of these two is out of balance.

    But actual nitrate poisoning cannot happen. The symptoms aquarists see are a sign of either ammonia poisoning or high nitrates.

    What Causes High Nitrite Levels In A Fish Tank?

    What causes high nitrites in fish tanks
    Lyretail molly (Poecilia latipinna)

    As you’ve just learned above, nitrite comes from ammonia. Therefore ammonia and nitrite levels are linked. If you’re seeing a rise in one, the other is implicated. So what causes ammonia and nitrite levels to rise?

    Death Of Beneficial Bacteria

    One common reason is that your beneficial bacteria count has dropped. These organisms live everywhere in your fish tank. Any functioning tank has them as a healthy part of your aquarium ecology.

    They are found in the highest numbers inside your filter’s biological media. As well as in your aquarium gravel or sand. Smaller numbers float freely in the water as well.

    Once you have established your tank with bacteria (one with cycled filters) they are very hardy. So what can cause your beneficial bacteria to die out?

    The most common reason is, unfortunately, the aquarium keeper. An established tank is not always a clean one. Maroon muck (mulm) can accumulate on the bottom and under the gravel.

    This brownish-red mulm is actually what colonies of healthy bacteria look like. But new aquarists might see that as gross dirt that should be removed.

    So they do a “spring cleaning” and scrub the tank. Remove all of the water, clean every inside surface of slime…Clean out the cycled filter, add brand-new filter media, and get the gravel looking spotless. Then they add all of the fish back into the tank.

    At this point, they often have many fish die. Cleaning out all of your beneficial bacteria resets the nitrogen cycle.Which causes ammonia and nitrite levels to rise very fast until the nitrifying bacteria can re-establish themselves. This is sometimes called new tank syndrome.

    Dead Fish Or Leftover Food

    The second way nitrite spikes occur is when a large amount of protein-rich organic matter starts to decay. One common source is a dead fish, shrimp, snail, or some other animal. If you catch a dead pet a few minutes after it dies, you won’t see sudden nitrite spikes.

    But if it is allowed to remain for several hours, or worse, several days, high nitrite is inevitable. First, you will have elevated ammonia levels since ammonia is the first stage in the nitrogen cycle.

    After it’s eaten by your first set of bacteria in your cycled filters, you have secondary nitrite spikes 1 to 3 days after the initial ammonia spike. That’s assuming you do nothing to counteract the ammonia that’s in your aquarium water.

    1 to 3 days after the high nitrite levels are detected, you may see nitrate levels rise a little. Your nitrate tests might not be sensitive enough to detect them, however.

    Remember, nitrate levels are less critical to adult fish health. A blip of 1 ppm of nitrite is significant on a test that goes from 0 to 5 ppm. But nitrate tests may measure a range of 0 to 40 ppm. An extra 1 ppm might not be noticeable.

    Signs Of High Nitrate Levels In Aquarium Water

    You won’t be able to detect signs of high nitrite levels in aquarium water just by looking. Nitrite has no smell or color.

    The only way to tell is to do semi-frequent tests using a nitrite test kit. I recommend water tests once per week, checking all three nitrogenous waste levels (ammonia, nitrite, nitrate). If your aquarium setup is a new tank, test it 2 to 3 times per week until it has a fully cycled aquarium filter.

    Also, test your water quality whenever you see signs of a problem. Besides the signs of nitrite poisoning outlined below, look for any signs of stress. Lack of appetite, diseases, and excess mucus production all indicate stress in aquarium fish.

    Signs Of Nitrite Poisoning In Fish

    Signs of nitrite poisoning in fish
    Guppy (Poecilia reticulata)

    Gauging the behavior of your fish and invertebrates is even more important. Here are the most common signs of nitrite toxicity:

    • rapid gill movement
    • lethargic behavior
    • lack of appetite

    Rapid Gill Movement

    You will see your fish breathing much faster when nitrite levels are too high. They are working hard to clear nitrite out of their system.

    The gills may even look bright red and inflamed as blood starts to collect there. Bleeding is rare but can occur if concentrations grow too high.

    Brown gills are often listed as a symptom of nitrite poisoning in many online articles. But that is actually a symptom of nitrate levels being too high. Nitrate has a strong affinity for blood hemoglobin. Stronger than oxygen, in fact.

    So nitrate ends up replacing oxygen in fish blood. That’s why it’s sometimes referred to as brown blood disease. By the way, this is similar to how carbon monoxide poisoning works in us humans.

    Lethargic Behavior

    As your fish struggle to breathe properly, lethargy sets in. They may lay on the bottom of the aquarium. Or spend all of their time at the surface, where oxygen levels are highest.

    Adding aeration to the tank will help reduce stress levels. But extra oxygen doesn’t counteract nitrite. You still need to address the root causes.

    Lack Of Appetite

    As nitrite levels grow worse, your fish may stop eating entirely. This stage is extra-serious since not taking in food further stresses their bodily functions.

    The fish lose the ability to flush nitrite from their systems. Death is likely in just a day or a few hours once they stop eating.

    Leftover food can also make your nitrite problem even worse. Since it is caused by decaying organic matter, uneaten food decays into more ammonia and nitrite.

    How Can Nitrite Levels Be Tested In An Aquarium?

    Since you can’t see or smell nitrite, you’ll need a water test kit to measure the exact concentration.

    Nitrite test strips are cheap and easy to find in local stores. But they are not very exact. I’d only recommend using them for testing the presence or absence of nitrite.

    For more exact measurements liquid test kits do a much better job. They allow you to determine the nitrite concentration with precision. Which is really important when dealing with sick or dying fish.

    What Can Happen If Nitrite Levels Are Not Lowered?

    If nitrite levels are not lowered, fish tank health will be compromised. Fish will show all of the above signs of nitrite poisoning. And as concentrations grow higher, they will start to die.

    Some fish are more resistant to high nitrite concentrations than others. Betta fish and livebearers are very hardy. But it will still cause them stress and eventually kill them.

    And once fish start to die, they will cause ammonia spikes as bacteria break down their bodies. Each ammonia spike will cause a corresponding nitrite spike in 1 to 3 days. Worsening the death spiral of your fish tank.

    Constant high nitrite levels also cause algae to grow. Algae love nitrite as a fertilizer; only ammonia is a better source of food for it. The decay of fish waste and leftover food also releases phosphates into the water. Which also fuels algae growth.

    Green aquarium water is often caused by high levels of nitrogenous waste products. Green spot algae, green hair algae, black beard algae, staghorn algae…All of these are signs that you need to retest your water quality.

    How Long Can Fish Live With High Nitrite In Their Aquarium Water?

    Freshwater fish will die in 1 to 3 days if high nitrite levels are not corrected. Moderate nitrite concentrations can be survived indefinitely. But the compound will cause constant stress. Interfering with growth, diet, reproduction, and disease resistance.

    Salt water fish can survive indefinitely at nitrite levels that would kill a freshwater fish in a day. The salt concentrations in the ocean neutralize the ability of nitrite to be poisonous.

    The most sensitive marine fish will die at around 60 ppm. But that’s many, many times higher than what would kill a freshwater fish.

    How To Prevent Nitrite Levels From Rising

    Harlequin rasbora (Trigonostigma heteromorpha)

    Nitrite levels rise due to organic matter decaying into ammonia. Which is then converted into nitrite by your helpful aquarium bacteria. If there is too much nitrite for the second set of bacteria to consume, it will collect.

    So prevention is truly the best medicine here. Nitrite can’t accumulate to poisonous levels unless ammonia levels are/were high. Regular aquarium maintenance will prevent nitrite from becoming an issue.

    Do weekly or biweekly water changes, depending on the number of fish you have. Check your biological filter media and replace it as needed. If any fish die, remove them right away.

    And never let uneaten food simply decay. Do micro water changes, sucking it up to keep it from elevating ammonia and nitrite amounts.

    Live plants are another good prevention strategy. If your bacteria can’t handle a sudden increase in ammonia or nitrite, they act as a second level of protection for your fish.

    Plants use nitrite as fertilizer. They prefer ammonia but use nitrite as well. They will also absorb nitrite before they do nitrate.

    Once plants consume nitrite as a fertilizer they will put it into their leaves and stems. You can then trim back plant growth. Effectively exporting it from your aquarium.

    Any yellowing or browning leaves should be trimmed and removed before they die. Dead plants will decay, releasing ammonia and nitrite back into the water.

    Fast-growing aquatic plants like water wisteria (Hygrophila difformis), hornwort (Ceratophyllum sp.), and Amazon sword plants (Echinodorus sp.) all do well at absorbing ammonia, nitrite, and nitrate. The more plants you have, the better your nitrite absorption capacity.

    Floating aquatic plants like duckweed (family Lemnaceae) do an even better job. Plus they provide shade, which keeps algae from taking over. Just beware of duckweed since it will take over the entire tank if you let it. Especially one with nitrite-rich water.


    Nitrite poisoning is very dangerous for freshwater fish tanks. If you have detectable levels, you need to act swiftly. Otherwise, fish may die in 1 to 3 days from high exposure. Even moderate levels demand action as it causes fish continual stress.

    Marine aquarists won’t have fish die from high nitrite levels. But they should still keep track of nitrite since it indicates the presence of ammonia and/or nitrate. Not to mention nitrite causes algae blooms in both freshwater and marine fish tanks.

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