Box for the dissolved oxygen O2 water test, 10.6 by 17.5 cm format, 40 x TEST stated on the packaging

Contents of the dissolved oxygen test: three reagent bottles, glass test tube with screw cap, syringe and colour reading chart.

Reading the colour chart of the dissolved oxygen test, held by hand at the water's edge

Dissolved oxygen test kit placed at the edge of a garden pond, box, reagent bottles and colour chart

Oxygen solubility curve in freshwater, from 0 to 35 degrees, at atmospheric pressure, with the typical summer temperature range of a garden pond

Dissolved oxygen water test in drops

Name: Dissolved oxygen water test in drops
Brand: Hardy Dewerse
Price: 14.9 EUR
Availability: InStock

14,90 €

4,96 € × 3 with no fees Klarna.

A lack of oxygen can kill the fish in a pond in a single summer night. This test measures the actual dissolved oxygen, from 1 to 10 mg/L, in aquariums, ponds, and aquaponics systems.

The difficulty lies in the sampling: the test tube must be filled by immersion, to the brim, and the slightest trapped bubble will skew the result upwards.

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The summary

What you receive: 3 reagent bottles, a glass test tube with a screw cap, a syringe and a colour chart.
What you measure: dissolved oxygen from 1 to 10 mg/L, in fresh water, in sea water and in ponds.
When to measure: first thing in the morning, before sunrise. This is the low point of the day.
How long it takes: 12 minutes per measurement, 10 of which are waiting time.
How to choose: this test is sold on its own, and oxygen is also included in the 13-in-1 aquarium test case and in the 10-in-1 pond test case.
What is sent to you: a 0.11 kg parcel, with the reagents in their original box.

Dissolved oxygen drop test, from 1 to 10 mg/L, for aquariums, ponds, and aquaponics

A rise in ammonium or nitrite develops over several days, whereas an oxygen deficit becomes dangerous within hours. A single night spent in the red zone of the colour chart, at 2 mg/L or below, can be enough to kill the fish in a pond where nothing seemed wrong the previous evening, because the level reaches its lowest point just before dawn, at a time when measurements are rarely taken. This test is therefore best carried out first thing in the morning, to capture the lowest reading of the day.

🌞 Warm water, less oxygen

The amount of oxygen that water can dissolve depends primarily on its temperature. Saturated fresh water holds approximately 11 mg/L of oxygen at 10 °C, and less than 8 mg/L at 30 °C. A heatwave therefore acts on your pond in two ways at once: it lowers the available ceiling while the heat accelerates the respiration of fish and bacteria, which consume more as a result. This is the mechanism described in the summer system maintenance guide.

Break the surface as soon as oxygen drops below 4 mg/L

Aim for 5 to 8 mg/L in a freshwater community aquarium, 5 to 10 mg/L in a lightly stocked planted tank, in marine water, and in a pond. Below 4 mg/L, break the water surface: an air pump and its diffusers raise the level within a few hours by renewing the water in contact with the air. Then monitor nitrite in the days that follow, as nitrifying bacteria require oxygen to convert ammonium.

🌱 Oxygen in aquaponics

In an aquaponic system, three populations breathe the same water: the fish, the nitrifying bacteria in the biofilter, and the plant roots. The bacteria need oxygen to convert ammonium into nitrate, so a shortage of oxygen often shows up in nitrification before it becomes visible in the fish. Aim for at least 5 mg/L in the fish tank as well as in the grow beds, and take your reading at the least agitated point, the far end of a gutter or the bottom of a media bed, rather than right next to the diffuser. The full start-up process for a system is described in the article Getting started with an aquaponic system.

An end-of-day reading will overestimate your oxygen level

Plants and algae produce oxygen whenever there is light, and consume it throughout the night without producing any. In a well-planted tank or pond, the concentration can even exceed saturation by 1 to 2 mg/L at the end of the photoperiod. The peak concentration is therefore reached in the late afternoon, and the minimum concentration just before sunrise, which means a reading taken at the end of the day gives the highest value without telling you anything about the lowest.

Garden pond in summer

Warm water, high biomass and short nights all combine. Green water pushes the nocturnal minimum even lower.

Aquaponics

The biofilter consumes continuously, on top of the fish. Without oxygen, nitrification stops.

Planted tank with CO2 injection

Oxygen levels drop at night, whilst gas injection sometimes continues.

After a power cut

A pump stopped for a few hours, a treatment, an algae bloom that dies off suddenly: each one can send oxygen levels plummeting within a day.

🫧 Filling without air bubbles

The sample for this test is not drawn with a syringe. Rinse the test tube, then fill it by immersion, right to the brim, and set it on a watertight surface. Add the 6 drops of the first reagent and the 6 drops of the second slowly, allowing the tube to overflow, then screw on the cap without trapping any bubbles. These two reagents chemically fix the oxygen present at the moment of sealing: a bubble trapped at this stage would introduce its own oxygen and your reading would be overestimated. After 30 seconds of agitation, the third reagent develops the colour, ranging from yellow to deep red, and any bubbles trapped when recapping the tube no longer have any effect on the result.

Technical specifications

The essentials

Parameter measured: Oxygen dissolved in the waterThe method chemically fixes the oxygen present at the moment of sampling, then reveals it through a colour change from yellow to dark red.
Measurement range: 1 to 10 mg/LIn freshwater, milligrams per litre is equivalent to ppm
Tests per kit: Approximately 40Each test uses 6 drops from each of the 3 bottles, making 18 drops in total.
Target values: 5 to 8 mg/L in fresh water5 to 10 mg/L in a lightly stocked planted tank, in seawater, and in a pond. The colour chart turns red at 2 mg/L and below
Method: Chemical fixation, colorimetric readingManganese salt, a strong base followed by an acid indicator: this is the principle behind laboratory oxygen titrations
Manufacturer's reference: JBL 2411200, PROAQUATEST rangeCode EAN 4014162241122

The procedure

Sampling: Test tube filled by immersionRight up to the brim, with no bubbles, and not with a syringe
The steps: Rinse, submerge, dose, shake, develop, readIn that order. The cap is screwed on while letting the test tube overflow.
Drops per reagent: 6 drops, three times6 drops of reagent 1, 6 of reagent 2, then 6 of reagent 3 after the first shake
Shaking and waiting: 2 times 30 s, then 10 min.Reading before the 10 minutes are up will underestimate the value
Total duration: Approximately 12 minutesHandling included
Reading: Test tube laid flat, 3 to 5 cm from the colour chartMoved back and forth over the white section until the best colour match is achieved

Security and parcels

Ranking: DangerThe three bottles are arranged
Main risks: Burns, eye injuriesH314 and H318 warnings on all three bottles, H373 and H412 for reagent 1, H317 and H334 for reagent 3
Precautions: Sink, well-ventilated room, out of reachNever smell a bottle or a test tube, and empty the treated test tube down the sink
Parcel weight: 0.11 kgPackaging included

To budget for separately

Reagent refill: Reagents only, ref. 2411300The glass test tube, its stopper, the syringe, and the colour chart can all be stored
What to correct with: An air pump and its diffusersBelow 4 mg/L in the early morning, aeration should be increased that same day

10 further specifications are available in the detailed view.

In the box

3 reagent bottles
1 screw-top glass test tube
1 graduated sampling syringe
1 white-backed colour reference chart
1 multilingual instruction leaflet, French included

Reference, EAN code, number of tests, protocol, recommended values by water type, and hazard statements taken from the manufacturer's data sheet and instruction manual on 05/09/2026. Saturation levels by temperature are from the FAO table.

Oxygen is not included in all test kits

This is a parameter not included in every test kit. Before buying a kit to measure it, check that it actually contains the relevant test.

Oxygen depletion occurs before sunrise At night, plants consume oxygen without producing any, and warm water holds less of it in solution. A reading taken in the afternoon therefore misses this minimum.

	Dissolved oxygen	Parameters covered (parameters)	Drop tests included (tests)	pH, KH, nitrites, nitrates and ammonium	GH, iron, phosphates, copper, and silicates	Reagent bottles (bottles)	Glass test tubes	Screw-cap test tube	Thermometer	Reagent refill sold separately	Price
Recommended Dissolved oxygen test onlyYour choice	Yes	1 parameters	1 tests	No	No	3 bottles	1	Yes	No	Yes	14,90 €
6-in-1 Aquarium Water Testing KitYour choice	No	6 parameters	5 tests	Yes	No	9 bottles	3	No	No	Yes	50,90 €
13-in-1 water testing kit for aquariumsYour choice	Yes	12 parameters	13 tests	Yes	Yes	23 bottles	12	Yes	Yes	Yes	119,90 €

Dissolved oxygen test onlyRecommended

What you're measuring

Dissolved oxygenYes

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

pH, KH, nitrites, nitrates and ammoniumNo

GH, iron, phosphates, copper, and silicatesNo

Equipment supplied

Reagent bottles (bottles)3 bottles

Glass test tubes1

Screw-cap test tubeYes

ThermometerNo

Budget

Reagent refill sold separatelyYes

6-in-1 Aquarium Water Testing Kit

What you're measuring

Dissolved oxygenNo

Parameters covered (parameters)6 parameters

Drop tests included (tests)5 tests

pH, KH, nitrites, nitrates and ammoniumYes

GH, iron, phosphates, copper, and silicatesNo

Equipment supplied

Reagent bottles (bottles)9 bottles

Glass test tubes3

Screw-cap test tubeNo

ThermometerNo

Budget

Reagent refill sold separatelyYes

13-in-1 water testing kit for aquariums

What you're measuring

Dissolved oxygenYes

Parameters covered (parameters)12 parameters

Drop tests included (tests)13 tests

pH, KH, nitrites, nitrates and ammoniumYes

GH, iron, phosphates, copper, and silicatesYes

Equipment supplied

Reagent bottles (bottles)23 bottles

Glass test tubes12

Screw-cap test tubeYes

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

The eight-step protocol, filling the test tube by immersion, and the oxygen saturation table as a function of temperature on the last page.

PDF · 1,5 Mo

Download

Estimate your
delivery time

0.12 kg parcel, dispatched within 48 hours by Colissimo or Chronopost home delivery, to be chosen at checkout. The reagents ship in their original box. The exact cost is shown in the basket.

Frequently asked questions

6 questions

6 key questions

Why are my fish coming up to breathe at the surface in the morning?

At what level should you start to worry?

How can oxygen levels be raised quickly?

Break the surface: an air pump, a waterfall, a jet directed upwards.
Lower the temperature if you can — cooler water dissolves more oxygen.
Stop feeding for 24 hours, as digestion consumes oxygen.
Remove anything decomposing on the bottom: dead leaves and food waste.

Surface agitation works faster than anything else, because it is the area of contact with the air that allows oxygen in, not the volume of bubbles.

Is an air pump enough for a pond?

It depends on the volume and the fish load, and the only way to find out is to measure at dawn before and after. A 600 L/h air pump makes a significant difference in 1,000 litres and very little in 10,000. What matters is not the stated air output but the movement it creates throughout the entire water column: a diffuser placed at the deepest point circulates water more effectively than two diffusers placed along the edges.

I still have the old two-reagent test; will the refill work with it?

Does the test work in a garden pond?

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Dissolved oxygen water test in drops

The summary