How to Lower Nitrite Levels in a Freshwater & Saltwater Aquarium on Time?

Nitrite can cause significant damage to aquatic inhabitants or even kill them.

So how do you protect them if an imbalance occurs?

Why is it that with goldfish nitrite levels can spike more often?

How to reduce said levels on time, without suffering fish losses?

Is there a silver bullet nitrite remover that can take care of the mess?

Let’s dive right in.

Before we start, I need to make sure that you understand how the nitrogen cycle works:

  1. Organic matter degrades to ammonia (NH3).
  2. Bacteria convert that ammonia to nitrites (NO2-).
  3. Other bacteria transform the nitrites in nitrates (NO3-).

With that out of the way, we can now answer the important questions.

How to promptly bring down nitrites in your freshwater aquarium?

Nitrites are the second most toxic byproduct in the nitrogen cycle of an aquarium.

They can be lethal to most freshwater fish if not handled on time. To promptly lower the high nitrite levels in your freshwater aquarium follow these exact steps:

1. Change 30% of the water.

With this initial step, you aim to replace part of the nitrite-rich water.

By physically diluting the concentration you will ease the negative effects on your livestock.

2. Add nitrifying bacteria

Since high nitrite levels are signaling an incomplete nitrogen cycle, this should be your second course of action.

If your ammonia reads 0, but your nitrites are high your fish tank lacks nitrite-oxidizing organisms.

There are many bottled bacteria products on the market.

Most of them are complete garbage, however, the reason being the use of cheap-grown land-based nitrifying bacteria.

They will essentially outcompete the not-well-established aquatic bacteria and then die out in the underwater conditions.

And then you’ll be left with another spike in nitrites and no one to handle that.

Luckily, after conducting my own experiments, I found four sound products that do actually work as marketed.

I also did a detailed comparison between two of them here (where I also include the correct ways to use them).

For the purpose of controlling nitrites, I strongly recommend using Tetra’s SafeStart Plus.

From what I’ve found, it arguably has more nitrite-converting bacteria than API’s Quick Start.

Look it up online (you can check it out here, on Chewy, a reputable online vendor specializing in pet supplies) or rush to your local fish store and buy it, as soon as you have the chance.

If you do end up in the fish store make sure to get a bottle that’s as recently-manufactured as possible.

Chewy supplies are usually fresh, but in some cases, you may get lucky in your LFS as well.

Dose accordingly, but in my experience, you should pour the whole bottle in anything larger than 30 gallons.

It is beneficial bacteria and it won’t do harm to your fish.

I’m sure you already figured that but here it is anyway- supply yourself with the bacteria first, so you can add it immediately after the initial water change.

Avoid changing the water for the next couple of days to let the bacteria establish itself.

Author’s note: Some will argue that you can use Seachem’s Stability in that situation as well.

Though Seachem is a reputable brand in the hobby, they refuse to name the bacteria used in their product.

This is what stopped me from ever bothering to test it, and therefore I can’t really recommend using it.

3. Set a filter

Filters are where most of the beneficial bacteria live.

By setting a filter you will do two things – improve the oxygen levels in the water and provide a living place for the bacteria.

The first will help your present fish to cope with the less oxygen in their blood, caused by the nitrite intoxication.

If you happen to have an established tank around you’re in luck.

Use SOME of its biological filtration in your nitrite-rich tank.

There you’ll have an already thriving bacterial colony that could immensely speed up the cycling process.

This is, again, a very effective step towards reducing the nitrites in your aquarium.

Bear in mind that borrowing too much media could impede your established tank’s cycle.

Note: For larger aquariums, perhaps, a smart decision would be to set a canister filter, as they have way more space for filter media and provide a very decent water turnover.

4. Add substrate from an established aquarium

The logic behind this is pretty much the same as in the previous step.

Bacteria also inhabit the substrate of your aquarium.

By adding SOME of that to your newer tank you improve their numbers.

How to (and should you) reduce nitrites in a saltwater fish tank?

In saltwater fish species nitrite is not toxic at all, or at least not notably toxic.

In freshwater aquariums, nitrite can be absurdly toxic.

This is because when it comes to uptake of proteins the nitrite will compete with chlorides in a fish’s body.

In freshwater, chloride is simply not enough to win the competition, which leads to intoxication.

The result is chloride depletion.

After the nitrites replace the chlorides, the balance of a number of biological mechanisms is disturbed.

However, when salt is present in the water the chloride levels skyrocket.

At chloride levels of 1.94% (or ~19,350 ppm) the nitrites can not outcompete them and poison a fish.

In some cases, a freshwater fish specimen will be a THOUSAND times more sensitive to nitrite than a saltwater fish.

Now, I’m not a chemist, but I do read a lot, to ensure the safety of my fish.

I learned all of that from this guy, who’s a doctor and knows what he’s talking about.

You’d need to have nitrite levels of above 25 ppm in a saltwater aquarium for fish to just feel sick, with no lasting effects.

If for some godawful reason you managed to raise them that much you should introduce nitrifying bacteria to the tank:

BIO-spira or API Quick Start are very useful here.

Either will speed up the nitrogen cycling. Still, if the nitrites are this high, there’s probably something large that is rotting or you just overdid it with the water changes.

Signs of nitrite poisoning going on in the aquarium?

Sometimes you can spot the symptoms of nitrite poisoning without the test kit.

Affected fish can show clear symptoms identifying the presence of nitrites.

The signs in fish behavior that signal high aquarium nitrites are the following:

  • darkened coloration of the gills
  • lethargic fish that are otherwise active
  • the death of an otherwise healthy fish
  • fish are gasping, with fast movements of the gills
  • constant striving to stay near the water surface
Testing your water should be your first course of action, whenever anything suspicious happens in your fish tank.

Protecting your fish from nitrite intoxication

The owners of freshwater aquariums should be concerned with their fishes’ health during elevated nitrite levels. To protect your freshwater fish from nitrite poisoning you should:

  1. Relocate the fish to an established tank.

    This is quite obvious, but some fishkeepers may underestimate the toxicity of nitrite.

    Relocating your fish is your best bet in ensuring their safety.

    However, I’m assuming that most of you that read this don’t really have an established tank laying around.

    Either this or it is full of large predatory fish that won’t really get along with the newcomers. In that case…

  2. Add rock salt to the water. By adding rock salt to the aquarium you enhance the present levels of chloride.

    The elevated chloride will prevent the nitrites from entering your fishes’ body through the gills.

    This will make them significantly more tolerant of the intoxication.

    Add 1 teaspoon of non-iodized salt per 10 gallons of water.

    However, do your research on your fish before that. Most all scaleless fish (corydoras, loaches, some other catfish), tetra species, most invertebrates, and aquatic plants are very salt-sensitive.

    You can use Aquarium-grade salt as well if you have that around.

    I do not recommend buying it for the sole purpose of treating nitrite poisoning as it is just overpriced regular rock salt.

    Kosher salt, however, is a sound choice.

  3. Add an air-stone.

    After the nitrites enter your fish’s bloodstream they bind to the hemoglobin, oxidizing it to methemoglobin.

    The latter does not carry oxygen, which can result in suffocating fish.

    This is often called Brown Blood disease.

    By adding an air-stone to your aquarium you aim at providing more oxygen to support the already O2-deprived fish.

Beware: Some users wrongly approach the issue by using Seachem Prime, as a nitrite remover.

Prime is a binder. It binds to the nitrites, converting them to other harmless molecules, which is labeled as “detoxifying”.

So far, so good, right? Well, not exactly.

If the bacteria fail to handle all the detoxified nitrites while the effect of Prime lasts it may become overwhelmed when the binder expires.

Another thing to consider is that if you’re using tap water for your freshwater aquarium it will most likely be disinfected through Chloramine instead of Chlorine by your water facility.

Chloramine is actually chlorine + ammonia. Seachem Prime removes the chlorine and detoxifies the ammonia.

But that’s actually extra ammonia released in the aquarium by the chemical bond of the two after Prime’s effect expires. And that’s extra amount converted to nitrite after…

After 48 hours of a nitrite-free aquarium, you will be at your starting point.

This puts many inexperienced fishkeepers in the situation where they continuously use Prime ($$$), claiming it helps the nitrogen cycle.

This is simply wrong and a waste of money.

High nitrite readings, but 0 ammonia?

The explanation behind this is very simple:

Whenever your aquarium has high levels of nitrites but no present ammonia, it means that it is halfway through its nitrogen cycle. Essentially, the ammonia-converting bacteria has established itself, while the nitrite-oxidizing colonies are still insufficient in numbers.

If your tank is brand new – this is not the time to add fish. If your tank has been established for a while and this happens – something hindered its nitrogen cycle.

Acceptable level of nitrites in an aquarium?

It is “acceptable” that we’re discussing here, so depending on your setup and fish species the answer can vary.

However, there’s a universally safe nitrite reading for any aquarium out there:

Generally, the safe level of nitrites in an aquarium is considered to be between 0 and 0.2 ppm (ml/g).

Bear in mind that some species of fish are more nitrite-tolerant than others.

What causes the progressive intoxication is the rapid ingestion of the poison (actually aiming for chlorides) through the gills.

Some fish species do not take up chloride from their gills, which makes them more tolerant to a nitrite spike.

Species such as the grass carp, mosquitofish, and ayu are quite sensitive and will perish before your nitrite readings manage to reach 1 ppm.

On the other hand, some species of catfish, the fathead minnow, the roach, and the tench fish may be able to withstand a reading of more than 4 ppm.

For saltwater fish, the lower barrier will be as much as 30 ppm of nitrite.

This is quite a lot and, honestly, very unlikely.

The honorable mention goes to the common eel, which can live comfortably for as much as 4 days in water that has over 1600 ppm of nitrites.

Impressive stuff.

How you got high nitrites in the first place?

Obviously, something lies behind all of this. Well, let me guess.

The significant spike in nitrite levels in the aquarium appeared after:

  • You recently added too many fish at once.
  • You recently started the aquarium (less than a month and a half ago).
  • You did a full-clean of the filters and substrate or changed them.
  • You have a heavily planted tank but forgot to prune and remove dead leaves lately.
  • You recently lost a fish but did not immediately remove its body.
  • You feed your fish more than twice a day.
  • You treated the water with an antibiotic without removing the biological filter first.
  • You put ice in the water in an attempt to cool it on a hot summer day.
  • You didn’t dechlorinate before a water change.
  • You have an overall small fish tank (under 10 gallons).

What all of these have in common is the imbalance between nitrites and beneficial bacteria.

When a complete nitrogen cycle is established in an aquarium the nitrites will be converted into nitrates as soon as they appear.

Adding more than 3 small fish at once can be the difference between overwhelmed bacteria and a healthy 20-gallon tank.

Goldfish are even messier, as are most freshwater eels and other larger carnivorous fish.

It is why the “inch per gallon” rule is and always has been utterly wrong.

I mean, fish stores have to make the most of any situation and budget, right?

Another way fishkeepers get rid of their beneficial bacteria by accident is cleaning the tank too much.

The biological filter should only be cleaned when really dirty, and it should be rinsed in an aquarium and not tap water.

Chlorine (or respectively Chloramine) in tap water is harmful to bad AND good bacteria.

And so are antibiotics.

Rotting organic matter can also cause a nitrite spike as it elevates ammonia.

And that includes anything from a dead fish to copious amounts of waste, food leftovers and even dead plant matter.

Lastly, sudden temperature changes can stress and eventually kill your beneficial bacteria.

Temperature changes should be implemented gradually.

So how to prevent nitrites building up in your aquarium?

The only sure way to stop nitrites from rising is to maintain a stable nitrogen cycle.

This is achieved by ensuring the well-being of your beneficial bacteria.

Overwhelming it with work will inevitably cause an unwanted imbalance in nitrites, nitrates, and even ammonia.

To prevent excessive levels of nitrite in your aquarium, follow these rules:

  1. Feed your fish once a day.

    Offering a meal once a day is a secure way to avoid overfeeding and spoiling leftovers.

    You can eliminate overfeeding by getting an automatic feeder.

    My heart lies with Eheim’s one (click the link to view it on

    It’s pretty reliable – just set it and forget it.

    You also get the added benefit of feeding your fish while you’re out of town.

  2. Dose food accordingly.

    You should not starve your fish, but giving them too much will result in water pollution.

  3. Get a bottom-feeder.

    Bottom feeders, such as rubber lipped plecos (really cheap, by the way) will scavenge the substrate for leftovers, cleaning them for you.

  4. Don’t clean all of your aquarium filters at once.

    Don’t clean the gravel too often as well, as these places harbor the nitrifying bacteria.

  5. Rinse your filter media with used aquarium water and not tap water.

    Chlorine in tap water is lethal to the good bacteria in a fish tank (click the link to learn how to naturally dechlorinate your water).

  6. Do not overstock your aquarium in short periods of time.

    Adding too many fish at once will overload your tank with organic waste.

    You will lose fish, money and your nitrogen cycle (time) in the process.

  7. Perform 25-30% on a weekly basis.

    This helps to dilute the contamination in the water, easing the work of the bacteria.

  8. Set up sufficient aeration.

    This will help the fish to get sufficient oxygen from the water, lowering the chance of nitrite poisoning.

  9. Increase the number of plants in the aquarium.

    Plants prefer to feed on ammonia over nitrites.

    However, if your ammonia-converting bacteria is in check and you lack the nitrite-converting one, plants will significantly reduce the damage by absorbing the excessive amounts.

    Wisteria, Water sprite, and duckweed can do that exceptionally well.

  10. Remove rotting flesh or food leftovers whenever you spot them.

    Often, degrading organic matter is what will cause a spike in nitrites.

  11. Reduce the incoming UV light, when the tank is cycling.

    Beneficial bacteria are sensitive to blue light, so during cycling reduce that to a minimum.

Over to you

High nitrites in the aquarium can be considered an emergency if you have live fish in it.

Still, diagnosing whether you’re at the phase of cycling or you broke the cycle by accident is crucial.

Finding the reason behind the suspiciously high levels is your final goal.

This way you can properly approach the issue and resolve it, without suffering losses.

Ask in the comments if you need more answers.

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