Cardboard box for the NO3 nitrate water test, labelled 40 X TEST

Contents of the NO3 nitrate test: two reagent bottles, two glass test tubes, syringe, double measuring spoon, colour comparator block, and colour chart

Nitrate reading: grey comparator block and two test tubes placed on the colour chart

The two reagents for the NO3 nitrate test, reagent 1 as a powder and reagent 2 as drops

Nitrate NO3 water test in drops

Name: Nitrate NO3 water test in drops
Brand: Hardy Dewerse
Price: 19.9 EUR
Availability: InStock

19,90 €

6,63 € × 3 with no fees Klarna.

This drop-based colorimetric test measures nitrate NO3 in aquariums, ponds, and aquaponic systems, using a colour comparator block and a reference sample. In freshwater, the recommended value should remain below 30 mg/L.

Nitrate is the final stage of the nitrogen cycle: a waste product to dilute in an aquarium, and a nitrogen feed for crops in aquaponics.

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The summary

In the box: 2 reagent bottles — one powder, one liquid — 2 glass test tubes, a sampling syringe, a double-ended measuring spoon, a comparator block and a colour chart.
What it measures: dissolved nitrate, across a nine-field colour chart, using a reference sample that cancels out the natural colour of the water.
How long it takes: 12 minutes per test, including 10 minutes of waiting time, with approximately 40 tests per kit.
Useful thresholds: 30 mg/L in a freshwater aquarium, 20 mg/L in marine water, 5 mg/L in a garden pond.
The one reading worth doing: test your top-up water on its own in a glass as well. Its nitrate content tells you whether a water change dilutes nitrate or actually adds more.
What gets sent to you: a 0.15 kg parcel, reagents in their original packaging, available for collection point or home delivery.

Nitrate drop test, 40 measurements, aquarium, pond and aquaponics

Nitrate is the final stage of the nitrogen cycle. Proteins break down into ammonium, ammonium becomes nitrite, nitrite becomes nitrate, and nitrification stops there. In a lightly planted aquarium, the plants absorb too little of it to compensate, so it accumulates until the next water change.

🔗 The final stage of the cycle

A nitrate level that rises week on week while ammonium and nitrite remain undetectable tells you two things at once. Your biological filter is working, since the bacteria are converting everything that reaches them, and your nitrogen consumers are not keeping pace with it. In a garden pond, this imbalance is most common in a heavily stocked koi pond with no base grow medium and no planted lagoon zone, or when garden fertilisers run off into the water. The nitrogen cycle follows the same course in an aquarium and in aquaponics, but the conclusions you draw from it differ depending on whether you are keeping fish only or also growing plants.

Nitrate high, ammonium and nitrite at zero

Nitrification is working, but the nitrogen produced is not being consumed sufficiently: check plants, growing surface, or reduce fish numbers.

High nitrate with ammonium

Two separate problems are occurring at once. Treat the ammonium first, as its toxicity is immediate, then correct the nitrate the following day.

Low nitrate in a planted tank

With plenty of plants and just a few small fish, nitrate becomes a deficiency factor and must be supplied via a nitrogen fertiliser.

Low nitrate in aquaponics

The lower leaves pale before the figures raise the alarm. Your fish load is low, the biofilter is slow, or the plants have taken everything up.

Shake the powder down to the grey residue before reading

Reagent 1 is a powder, and the accuracy of the result depends on thorough mixing. Add the large measure of reagent 1 followed by the 6 drops of reagent 2, close the test tube, and shake vigorously until only a few grey powder residues remain. Then allow the full 10 minutes' rest, if you skip this, the colour will not develop completely and you will read a value that is too low. Use the supplied syringe to measure both 10 ml samples rather than estimating by eye.

🥬 A nitrogen ration for your crops

The same figure can be read in two opposite ways. The aquarist measures nitrate to know when to siphon, because in their tank it is a waste product to be removed. If you are growing plants, you measure it to find out whether your lettuces have enough nitrogen for growth: nitrate here is a nitrogen ration that your fish produce and your plants consume. A figure that keeps rising despite well-stocked grow beds signals that the balance between source and consumers has broken down.

Plan to reorder whichever bottle empties first

Order the reagent refill now, because on a working installation, nitrate is measured every week and its bottle will be the first you empty. The glassware, the measure, the comparison block, and the colour chart can all be kept from one purchase to the next. This way you have a full season of weekly readings without having to wait for a new order just when levels start to rise.

When levels exceed what your plants can absorb, several approaches can be combined. Regular partial water changes dilute the concentration, a nitrate-reducing filter medium removes a portion, and you can also increase the planted area or reduce the fish stock. In a garden pond, sufficient substrate for rooted plants contributes too. In aquaponics, adding a grow bed is almost always preferable to draining water loaded with nitrogen your plants could have taken up. For the balance between fish and crops, see also the article Getting started with an aquaponic system.

🚰 Your top-up water matters

Your tap water already contains nitrate. The French regulatory limit for drinking water is set at 50 mg/L, which means perfectly compliant water can contribute a significant amount, and the actual level depends entirely on your local supply. Fill a glass from the tap and measure that water on its own: this reading, which only needs to be done once, tells you whether each water change dilutes the nitrate in your tank or adds to it.

🎨 One colour chart, three environments

The colour chart does not carry a single scale but two rows of figures and three colour bands. The top row is for freshwater and pond use, the bottom row for marine water, because high salinity dampens the colour development of the reaction: the same shade corresponds to a higher concentration. Above each field, a pictogram identifies the environment, a fish for the freshwater aquarium, a koi for the pond, a starfish for the reef, and a green, yellow, or red band indicates whether action is needed. Always read the line for your own environment, as the same value may be green for an aquarium and already yellow for a pond.

These stricter thresholds for ponds come down to algae. A high nitrate level only encourages their growth when the water also contains phosphate, and a pond exposed to sunlight and fluctuating temperatures reacts more quickly than an aquarium. This is why green water is rarely resolved without measuring phosphate at the same time as nitrate, from the same sample. Other approaches, from shading to marginal plants, are covered in the article on green water in ponds.

Technical specifications

The essentials

Parameter measured: Nitrate, expressed as NO3The final stage of the nitrogen cycle, produced by bacteria from nitrite
Measurement range: 0.5 to 200 mg/LNine-field colour chart, graduated up to 220 mg/L for saltwater
Tests per kit: Approximately 40
Recommended thresholds: No more than 30 mg/L in freshwater20 mg/L in a marine aquarium, 5 mg/L in a garden pond
Method: Powder, drops and block comparatorThe blank tube cancels the natural tint of the water, even when it is heavily coloured
Reference: JBL 2412500, PROAQUATEST rangeCode EAN 4014162241252

The procedure

Water volume: 2 times 10 mlTwice as long as for ammonium and nitrite. One treated test tube, one control test tube
Reagents to dose: 1 level measure and 6 dropsLarge end of the double-ended measuring spoon for reagent 1, in powder form
The steps: Rinse, fill, dose, shake, wait, readIn that order. Vigorous shaking is the step most often cut short, and the accuracy of the result depends on it.
Waiting time: 10 minutesFull colour development time. Reading earlier will give a lower value
Total duration: Approximately 12 minutesHandling included
Testing frequency: 1 time per weekAlways at the same time, and at least 24 hours after a water change

Security and parcels

Ranking: DangerThe two reagent bottles are classified
Main risks: Burns, eye injuries, allergic reactionH314, H318 and H317 hazard statements. H341, suspected genetic defects, for reagent 2
Precautions: Gloves, sink, out of reachEmpty the treated test tube down the sink, never into the tank: H411 and H412 statements, aquatic toxicity
Parcel weight: 0.15 kgPackaging included

To budget for separately

Reagent refill: Reagents only, ref. 2412600The glassware, the syringe, the measuring scoop, the comparator block and the colour chart can all be kept
To read alongside: PhosphateAn algae bloom requires both factors in the same water: nitrate alone does not account for it

10 further specifications are available in the detailed view.

In the box

2 reagent bottles, one in powder form and one in drops
2 glass test tubes with screw caps
1 graduated sampling syringe
1 double-ended measure, large and small tip
1 grey comparator block with reading slot
1 colour chart and 1 multilingual instruction leaflet, French included

Reference, EAN code, kit composition, number of tests, protocol, thresholds by medium and hazard statements taken from the manufacturer's product data sheet and instructions for use, 19/09/2026. The range and nine fields are read from the colour chart supplied. The 50 mg/L limit for drinking water is the French regulatory quality standard.

Nitrate alone, or the full nitrogen cycle?

Same drop reagents, same colour comparator block, same manufacturer. What changes is the number of questions the kit can answer.

The nitrate test alone, or a full kit? Two questions about your setup, and the relevant row highlights automatically.

	Parameters covered (parameters)	Drop tests included (tests)	Nitrate NO3	Ammonium, nitrites, pH and carbonate hardness	Phosphates, iron, copper, oxygen and silicates	Glass test tubes	Laboratory comparator block	Thermometer	Reagent refill sold separately	Price
Recommended Nitrate NO3 test onlyYour choice	1 parameters	1 tests	Yes	No	No	2	Yes	No	Yes	19,90 €
6-in-1 Aquarium Water Testing KitYour choice	6 parameters	5 tests	Yes	Yes	No	3	Yes	No	Yes	50,90 €
14-in-1 water analysis kit for aquariumYour choice	14 parameters	13 tests	Yes	Yes	Yes	12	Yes	Yes	Yes	119,90 €

Nitrate NO3 test onlyRecommended

What the kit covers

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

Nitrate NO3Yes

Ammonium, nitrites, pH and carbonate hardnessNo

Phosphates, iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes2

Laboratory comparator blockYes

ThermometerNo

What needs replacing

Reagent refill sold separatelyYes

6-in-1 Aquarium Water Testing Kit

What the kit covers

Parameters covered (parameters)6 parameters

Drop tests included (tests)5 tests

Nitrate NO3Yes

Ammonium, nitrites, pH and carbonate hardnessYes

Phosphates, iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes3

Laboratory comparator blockYes

ThermometerNo

What needs replacing

Reagent refill sold separatelyYes

14-in-1 water analysis kit for aquarium

What the kit covers

Parameters covered (parameters)14 parameters

Drop tests included (tests)13 tests

Nitrate NO3Yes

Ammonium, nitrites, pH and carbonate hardnessYes

Phosphates, iron, copper, oxygen and silicatesYes

Equipment supplied

Glass test tubes12

Laboratory comparator blockYes

ThermometerYes

What needs replacing

Reagent refill sold separatelyYes

Parameters covered, equipment supplied and kit contents recorded from the three product pages on 05/09/2026. Kits compared: 6-in-1 case and 14-in-1 case.

Documents & manuals

Official manufacturer's instructions, in French

Six-step protocol, volumes and dosage of both reagents, recommended thresholds for fresh water, salt water, and ponds, plus explicit mention of nitrate becoming a deficiency factor in a heavily planted tank.

PDF · 1,5 Mo

Download

Estimate your
delivery time

Parcel of 0.12 kg, dispatched to a collection point or by Colissimo to your home address, as chosen at checkout. The reagents are sent in their original box, powder and drops kept separate. The exact amount is shown at checkout.

Frequently asked questions

8 questions

6 key questions, 8 in the detailed view

What is the nitrate level that should not be exceeded?

My nitrates won't drop despite water changes — why?

Do nitrates really cause algae to proliferate?

My test reads zero — is that good or a bad sign?

In a pond or lightly planted tank: this is the desired result — the ideal value in a pond is not even measurable.

In a heavily planted tank or in aquaponics: this is a warning sign. With many plants and few fish, nitrate becomes a deficiency factor and must be supplemented with a nitrogen fertiliser. A persistent zero will be accompanied by pale lower leaves, and the root cause lies upstream: too few fish being fed, or a biofilter that has not yet fully established.

How to bring nitrate levels down when water changes are no longer enough

Test your fill water. If it already has a high concentration, no water change will bring the level below that of the fresh water itself.
Change water in meaningful volumes. Replacing a third of the volume reduces the gap with fresh water by a third — no more.
Add nitrogen consumers. Fast-growing plants, expanded growing surfaces and floating plants are the only lever that works continuously.
Reduce the source. Keep fewer fish, or feed them more precisely. The typical case is a heavily stocked koi pond with no planted zone.
Check phosphate at the same time. If your concern is algae rather than the number itself, phosphate matters just as much.

Why is the first reagent a powder?

Because the reaction first reduces the nitrate, and the reducing agent keeps better in dry form than dissolved. That is why a double measure is used rather than a simple dropper: one level large measure of reagent 1, then 6 drops of reagent 2.

A practical consequence, and the most common cause of an incorrect result: the test tube must be shaken vigorously until only grey powder residue remains. Incompletely dissolved powder produces a pale colour and therefore an underestimated reading.

Is the nitrate reagent suspected of being mutagenic?

Both bottles are classified under the Danger label: H314 and H318, causes severe skin burns and serious eye damage, and H317, may cause an allergic skin reaction. Reagent 2 also carries the statement H341, suspected of causing genetic defects.

Three precautions follow from this: wear gloves or wash your hands immediately, avoid any contact with the eyes, and dispose of the treated test tube contents down the sink — never into the tank — on account of the H411 and H412 statements, one per reagent, for toxicity to aquatic organisms. Store the kit out of reach: child-resistant caps are no substitute for a locked cupboard.

Can you rely on a reading taken immediately after a water change?

No, it tells you nothing reliable: the result is merely an average between the old water and the new. Wait 24 hours of circulation, and always measure at the same point in the week.

Keep two sets of readings in the same notebook — the value from the tank and the value from the top-up water. It is the difference between the two, not either value alone, that shows whether your system is accumulating nitrogen or receiving it from the mains.

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Nitrate NO3 water test in drops

The summary