Box of the nitrite NO2 drop water test, on a white background

Contents of the nitrite kit: grey comparator block, two glass test tubes, syringe, two reagent bottles and colour chart

Nitrite reading: comparator block placed on the nine-field colour chart

The two reagent bottles from the nitrite test, numbered 1 and 2

Nitrite NO2 water test in drops

Name: Nitrite NO2 water test in drops
Brand: Hardy Dewerse
Price: 13.9 EUR
Availability: InStock

13,90 €

4,63 € × 3 with no fees Klarna.

Nitrite is, along with ammonia, one of the two toxic compounds in a new tank. It appears after the ammonium, when the first family of bacteria is already active and the second is not yet established, and it blocks the transport of oxygen in your fish's blood.

This test measures it from 0.025 to 1 mg/L after 5 minutes, across nine colour fields, in freshwater as in seawater.

Water test

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The summary

What you receive: 2 reagent bottles, 2 glass test tubes, a syringe, a grey comparator block and a colour chart.
What nitrite does: it oxidises haemoglobin and blocks oxygen transport. A fish can suffocate in perfectly oxygenated water.
The threshold that matters: 0.5 to 1 mg/L can already be lethal depending on the species, and fry are more sensitive to it than adults.
How long it takes: 7 minutes in total, including 5 minutes waiting for the colour to develop.
How to choose: this is the bottle you empty first in a test kit, because it's the one you reach for most often.
What we send you: a parcel weighing 0.15 kg, reagents in their original packaging.

Nitrite drop test, from 0.025 to 1 mg/L, read in 5 minutes, with a target of zero

Nitrite is invisible and odourless. It always appears at the same point: a few days after ammonium, once the bacteria that convert ammonium into nitrite have established themselves but before those that convert nitrite into nitrate are sufficiently numerous. This is the nitrite spike, and it accounts for a large proportion of the losses seen during the first 3 weeks of a new tank.

🩸 Nitrite and haemoglobin

Nitrite uses the chloride ion transporter at the gill surface, concentrates in the plasma to up to 60 times the level in the water, and then oxidises the iron in haemoglobin. This converts it to methaemoglobin, which can no longer bind oxygen, and the blood takes on a brown tint. Your fish may therefore gasp at the surface and suffocate in oxygen-saturated water, because the oxygen present in the water is no longer carried to their tissues.

Because nitrite enters through the chloride transporter, dissolved chloride competes directly with it. Adding salt is therefore the only measure that protects your fish without changing anything about the nitrite content of the water. Relative to the value read by this test, expressed as NO2, the benchmark used in fish farming is around 2.5 mg/L of chloride per mg/L of nitrite for cyprinids, and around 5 for salmonids, which are more sensitive. Use a non-iodised salt with no anti-caking agent, dissolved separately, and never in an invertebrate tank or in a circuit feeding growing beds.

Sensitivity varies enormously depending on the species

Depending on the species, concentrations between 0.5 and 1 mg/L can already be lethal, and marine fish as well as fry are more sensitive than adults. Trout, salmon, and catfish are the most vulnerable; carp, tench, and eels are far more tolerant. A level that a koi carp can survive may therefore kill a trout in the same pond.

📈 Where your cycling stands

Nitrite rising, ammonium falling

This is the normal sequence of cycling. Measure every day and do not introduce fish.

Both tops together

Cycling has not started or has just been interrupted: filter rinsed under the tap, pump switched off.

Nitrite returning in an established tank

This situation is never normal; the usual causes are a sudden addition to the stock, overfeeding, a dead fish, or a blocked filter.

In aquaponics

A value readable on an established system indicates that your biofilter is undersized for the fish load.

Read nitrite alongside the ammonium result from the same day

If you are starting a tank or a system, use the ammonium test alongside this one. Nitrite alone does not tell you where your cycling stands, whereas both values read together do: ammonium falling while nitrite rises shows that the first bacterial colony is established. You will know whether to wait before adding fish, rather than finding out from their behaviour.

The only acceptable value is zero. Unlike nitrate, there is no tolerable range: any readable value indicates that nitrification is not converting all the nitrite produced, and the first course of action is a water change of around half the volume, while the bacteria recolonise the filter. The actions that trigger a spike, from rinsing a filter under the tap to introducing fish all at once, are covered in our article on common mistakes.

❄️ Nitrite when the water cools down

In autumn, the activity of nitrifying bacteria slows with the water temperature, while the fish are still receiving their full ration. A feed too rich in protein then produces more ammonia than the biofilter can convert, and nitrite levels rise dangerously in a pond that had remained stable all summer. Switch to a winter food, higher in lipids and lower in protein, as soon as the water begins to cool, as explained in the article on feeding fish in winter. An aquaponic system goes through the same transition, and one reading per week is sufficient to monitor it.

The test also works on cold water, but the colour develops more slowly, sometimes with a significant delay. Store the reagents at room temperature, between 20 and 25 °C, and leave your sample to stand for approximately 2 hours in a heated room before testing. If you obtain two conflicting readings taken at different temperatures, take the higher value as your reference.

Technical specifications

The essentials

Parameter measured: Nitrite, expressed as NO2The intermediate between ammonium and nitrate in the nitrogen cycle
Measurement range: 0.025 to 1 mg/LThe target is zero: unlike nitrate, no readable value is comfortable
The colour chart fields: 0,01 · 0,025 · 0,05 · 0,1 · 0,2 · 0,4 · 0,6 · 0,8 · 1,0Nine values, from yellow to pink. The first field, 0.01, falls below the measurement range
Danger threshold: 0.5 to 1 mg/L depending on the speciesMarine fish and fry are more sensitive than adults
Tests per kit: Approximately 50It is the first bottle you empty in a test kit, because it is the routine cycling test.
Manufacturer's reference: JBL 2412300, PROAQUATEST rangeCode EAN 4014162241238

The procedure

Water volume: 2 times 5 mlOne treated sample and one untreated blank, side by side in the comparator block
The steps: Rinse, fill, add reagent 1, add reagent 2, wait, readAdd 5 drops of each reagent, swirling after each addition
Waiting time: 5 minutesFull colour development time
Total duration: Approximately 7 minutesHandling included
Reading: In the comparison blockThe grey block compensates for the natural colour of the water, including peat
Environments covered: Freshwater and saltwaterOne measurement per day for 3 weeks on a new freshwater tank, one per week in saltwater

Security and parcels

Ranking: DangerBoth bottles are classified, and reagent 2 is flammable
Main risks: Burns, eye injuriesH314 and H318 hazard statements for reagent 1, H226 and H319 for reagent 2
Precautions: Sink, well-ventilated room, out of reachChild-safety caps on both bottles
Parcel weight: 0.15 kgPackaging included

To budget for separately

The matching test: An ammonium testThe two readings taken together show how far along your cycling is
Reagent refill: Reagents only, ref. 2412400The glassware, comparator block and colour chart can be kept

10 further specifications are available in the detailed view.

In the box

2 reagent bottles with child-resistant caps
2 glass test tubes with screw caps
1 graduated sampling syringe
1 grey laboratory comparator block
1 colour chart with 9 colour fields
1 multilingual instruction leaflet, French included

Reference, EAN code, kit composition, number of tests, protocol, mortality threshold and hazard statements taken from the manufacturer's product sheet and instructions on 05/09/2026. The range and nine fields are read from the colour chart supplied. The chloride-to-nitrite ratios, converted to NO2 from the EIFAC recommendation expressed as nitrous nitrogen, and the plasma concentration factor are taken from aquaculture veterinary literature.

Nitrite alone, or the entire nitrogen sequence?

Nitrite comes after ammonium and converts into nitrate. The question is whether you want to see one stage of the chain or the whole sequence.

The nitrite peak occurs in the middle of the cycle It rises as ammonium falls, and it can be read on its own or as part of the full NH4, NO2, NO3 sequence. What you need depends on where you are in the process.

	Parameters covered (parameters)	Drop tests included (tests)	Nitrite NO2, the most toxic stage	Ammonium NH4, the preceding stage	Nitrate NO3, the next stage	pH and KH, which govern bacterial activity	Iron, phosphates, copper, silicates, oxygen	Reagent bottles (bottles)	Glass test tubes	Laboratory comparator block	Thermometer	Reagent refill sold separately	Price
Recommended Nitrite test onlyYour choice	1 parameters	1 tests	Yes	No	No	No	No	2 bottles	2	Yes	No	Yes	13,90 €
6-in-1 Aquarium Water Testing KitYour choice	6 parameters	5 tests	Yes	Yes	Yes	Yes	No	9 bottles	3	Yes	No	Yes	50,90 €
14-in-1 water analysis kit for aquariumYour choice	14 parameters	13 tests	Yes	Yes	Yes	Yes	Yes	23 bottles	12	Yes	Yes	Yes	119,90 €

Nitrite test onlyRecommended

The nitrogen cycle

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

Nitrite NO2, the most toxic stageYes

Ammonium NH4, the preceding stageNo

Nitrate NO3, the next stageNo

pH and KH, which govern bacterial activityNo

Iron, phosphates, copper, silicates, oxygenNo

Equipment supplied

Reagent bottles (bottles)2 bottles

Glass test tubes2

Laboratory comparator blockYes

ThermometerNo

Budget

Reagent refill sold separatelyYes

6-in-1 Aquarium Water Testing Kit

The nitrogen cycle

Parameters covered (parameters)6 parameters

Drop tests included (tests)5 tests

Nitrite NO2, the most toxic stageYes

Ammonium NH4, the preceding stageYes

Nitrate NO3, the next stageYes

pH and KH, which govern bacterial activityYes

Iron, phosphates, copper, silicates, oxygenNo

Equipment supplied

Reagent bottles (bottles)9 bottles

Glass test tubes3

Laboratory comparator blockYes

ThermometerNo

Budget

Reagent refill sold separatelyYes

14-in-1 water analysis kit for aquarium

The nitrogen cycle

Parameters covered (parameters)14 parameters

Drop tests included (tests)13 tests

Nitrite NO2, the most toxic stageYes

Ammonium NH4, the preceding stageYes

Nitrate NO3, the next stageYes

pH and KH, which govern bacterial activityYes

Iron, phosphates, copper, silicates, oxygenYes

Equipment supplied

Reagent bottles (bottles)23 bottles

Glass test tubes12

Laboratory comparator blockYes

ThermometerYes

Budget

Reagent refill sold separatelyYes

Parameters covered, equipment supplied and kit contents noted from the three product pages on 05/09/2026.

Documents & manuals

Official manufacturer's instructions, in French

Six-step protocol, number of drops, colour development time, mortality threshold by species, and what to do if a reading is high.

PDF · 3,1 Mo

Download

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delivery time

Parcel weight 0.12 kg, dispatched to a collection point or by Colissimo to your home address — your choice at checkout. The reagents travel upright in their original box. The exact cost is shown in the basket.

Frequently asked questions

6 questions

6 key questions

How long does a nitrite spike last?

My fish are breathing at the surface even though the water is well oxygenated — why?

Because the problem lies not in the quantity of oxygen but in its transport. Nitrite oxidises the iron in haemoglobin, converting it to methaemoglobin, which can no longer bind oxygen.

Your fish is therefore gasping in the middle of fully oxygenated water. This is the one situation where increasing aeration makes almost no difference: the nitrite level must be reduced, and the gills protected with chloride in the meantime.

At what level does nitrite become lethal?

How do you bring nitrite levels down quickly?

Change approximately half the volume, using water with a pH close to that of the tank.
Add chloride in the form of non-iodised salt, dissolved separately — never with invertebrates or plants present.
Stop feeding for 2 days and remove anything that is decomposing.
Leave the filter alone: that is where the missing bacteria are establishing themselves.

Salt does not reduce the nitrite level in the water — it simply prevents it from entering the fish.

The spike has passed — can I add my fish now?

In aquaponics, should you also aim for zero?

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Nitrite NO2 water test in drops

The summary