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: 23.0 USD
Availability: InStock

$23.00

$7.66 × 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 and one drops, 2 glass test tubes, a syringe, a double measuring spoon, a comparator block and a colour chart.
What it measures: dissolved nitrate, on a colour chart of nine fields, with a control sample that cancels out the water's own tint.
How long it takes: 12 minutes per measurement, 10 of them waiting, and around 40 measurements per kit.
The useful thresholds: 30 mg/L in a freshwater aquarium, 20 mg/L in sea water, 5 mg/L in a garden pond.
The one-off reading to take: also measure your top-up water, on its own in a glass. Its content shows whether a water change dilutes the nitrate or adds to it.
What is sent to you: a 0.15 kg parcel, reagents in their original box, by Colissimo or Chronopost to your door.

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 from one week to the next while ammonium and nitrite stay undetectable tells you two things at once. Your biological filter is working, since the bacteria convert everything that reaches them, and whatever consumes the nitrogen is not keeping pace with it. In a garden pond, this imbalance is most often found in a heavily stocked koi pond with no bottom substrate or planted lagoon zone, or when garden fertiliser runs off into the water. The nitrogen cycle works in the same way in an aquarium and in aquaponics, but the conclusion you draw from it differs depending on whether you are only keeping fish or growing crops as well.

Nitrate high, ammonium and nitrite at zero

Nitrification is working, but the nitrogen produced is not being consumed enough: add plants or growing surface, or reduce the fish population.

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

Pale lower leaves go with this figure. The fish stocking is too low, the biofilter is not yet established, or the plants have taken up everything.

Shake the powder down to the grey residue before reading

Reagent 1 is a powder, and the accuracy of the result depends on shaking. Add the large measuring spoon of reagent 1, then the 6 drops of reagent 2, close the test tube and shake vigorously until only traces of grey powder remain. Then leave it to stand for the full 10 minutes, otherwise the colour is not fully developed and you will read a value that is too low. Draw up the two lots of 10 ml with the syringe supplied 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 for them it is a waste product to be removed. You, as a grower, measure it to know whether your lettuces have enough nitrogen for their growth: here nitrate is a nitrogen ration that your fish produce and your plants consume. A figure that rises despite well-stocked grow beds signals that the balance between the source and the consumers has been lost.

Plan to reorder whichever bottle empties first

Order the reagent refill now, because on a system that is growing crops, nitrate is measured every week and its bottle is the one you will empty first. The glassware, the measuring spoon, the comparator block and the colour chart, on the other hand, carry over from one purchase to the next. That way you have a whole season of weekly readings in hand, without waiting for a delivery just as the level starts to climb.

When the level exceeds what your crops take up, the remedies add up. Regular partial water changes dilute it, a nitrate-reducing filter medium removes part of it, and you can also increase the planted area or reduce the fish population. In a garden pond, a sufficient bottom substrate for the plants helps too. In aquaponics, adding a grow bed is almost always better than discarding water loaded with nitrogen that 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

Tap water most often contains nitrate. The French regulatory limit for drinking water is set at 50 mg/L, so a perfectly compliant water can supply a lot, and the level depends entirely on your commune. Fill a glass at the tap and measure that water on its own: this reading, which you only need to take once, shows whether each water change dilutes the nitrate in the 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 water, the bottom one for seawater, because high salinity dulls the colour of the reaction: the same shade corresponds to a higher concentration there. Above each field, a pictogram shows 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 you need to act. Always read the line for your own environment, because the same value can be green for the aquarium and already yellow for the pond.

These stricter guidelines for the pond come down to algae. A high nitrate level only encourages their growth when the water also contains phosphate, and a pond exposed to the sun and to changing temperatures reacts faster than an aquarium. This is why green water is rarely sorted out without measuring phosphate at the same time as nitrate, on the same sample. The other levers, from shading to marginal plants, are gathered 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	$23.00
6-in-1 Aquarium Water Testing KitYour choice	6 parameters	5 tests	Yes	Yes	No	3	Yes	No	Yes	$59.00
13-in-1 water testing kit for aquariumsYour choice	12 parameters	13 tests	Yes	Yes	Yes	12	Yes	Yes	Yes	$138.00

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

13-in-1 water testing kit for aquariums

What the kit covers

Parameters covered (parameters)12 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 taken from the three product pages on 05/09/2026. Product pages compared: 6-in-1 case and 13-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

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Estimate your
delivery time

0.12 kg parcel, dispatched within 48 hours by Colissimo or Chronopost home delivery, your choice at checkout. The reagents are sent in their original box, with the powder and drops kept separate. The exact amount is shown in your basket.

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 a lightly planted tank: this is the result you are after.

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

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

Test your top-up water. If it is already loaded, no water change will bring the level below its own.
Change water in useful volumes. A third of the volume reduces the gap with fresh water by a third, no more.
Add nitrogen consumers. Fast-growing plants, growing surface and floating plants are the lever that works continuously.
Reduce the source. Keep fewer fish, or give a more measured ration. The typical case is the heavily stocked koi pond with no planted area.
Look at phosphate at the same time. If your aim is to limit algae, it matters as much as nitrate.

Why is the first reagent a powder?

Because the reaction first reduces the nitrate, and the reducing agent keeps better dry than dissolved. This is the reason for the double measuring spoon rather than a simple dropper: one large level spoonful of reagent 1, then 6 drops of reagent 2.

A practical consequence, and the main cause of a false result: you must shake the test tube vigorously until only grey powder residues remain. An incompletely dissolved powder leaves a pale colour, and therefore an underestimated value.

Is the nitrate reagent suspected of being mutagenic?

Both bottles are classified under the signal word Danger: H314 and H318, severe skin burns and serious eye damage, and H317, skin allergy. Reagent 2 also carries the H341 statement, suspected of causing genetic defects.

Three precautions follow: gloves or immediate hand washing, no contact with the eyes, and emptying the treated test tube into the sink and never into the tank, because of the H411 and H412 statements, one per reagent, for toxicity to aquatic organisms. Store the box up high: child-resistant caps are no substitute for a closed cupboard.

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

No, it describes nothing stable: the result is merely an average between the old water and the new. Allow 24 hours of mixing, and always measure at the same point in the week.

Keep two series in the same notebook, the tank value and that of the top-up water. It is the gap between the two, and not the 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