Box for the CO2 drop water test, water analysis range, cardboard format for freshwater aquariums

Contents of the CO2 water test: two reagent bottles, graduated test tubes, sampling syringe and two-box colour chart

Two reading scales for dissolved CO2 in mg per litre: the manufacturer's scale for plants and the aquaculture literature scale for fish.

Curves of dissolved CO2 and pH over twenty-four hours in a planted tank, moving in opposite directions between day and night.

Dissolved CO2 water test in drops

Name: Dissolved CO2 water test in drops
Brand: JBL
Price: 13.9 EUR
Availability: InStock

13,90 €

4,63 € × 3 with no fees Klarna.

Dissolved CO2 feeds your plants, but in excess it interferes with your fish's ability to breathe. This test gives you the result directly in mg/L: count the drops of the second reagent until the colour shifts to pink, and each drop represents 2 mg/L.

The alternative method — the table that cross-references pH and KH — assumes that the acidity of your water comes from carbon dioxide. A titration makes no such assumption.

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The summary

What you measure: the CO2 actually dissolved in the water, read directly in mg/L, without going via pH or KH.
What you receive: 2 bottles of reagents, a graduated plastic test tube up to 20 ml, and a two-cell colour chart.
What the figure means: aim for 15 to 30 mg/L, a range that is safe for your fish, with 20 to 25 mg/L for optimum growth.
How long it takes: 3 minutes, with no waiting time: the colour is assessed drop by drop.
How to choose: the two test kits derive CO2 from a table; this test measures it directly in the water.
What is sent to you: a parcel weighing 0.13 kg, reagents in their original box.

Drop titration test, 1 drop = 2 mg/L of CO2, for freshwater

Dissolved carbon dioxide is the primary nutrient for aquatic plants, and it is also a gas that, in excess, prevents fish from expelling the carbon dioxide produced by their own respiration. The range suitable for both plants and fish is narrow, which means it needs to be adjusted based on a numerical reading rather than by eye.

📊 The pH and KH chart

The most widely used method is indirect: you measure the pH, you measure the KH with a carbonate hardness test, you cross-reference the two on a table and read off the CO2. It rests on a rarely stated assumption: that all the acidity in your water comes from dissolved carbon dioxide. Add peat, catappa leaves, wood leaching tannins, or a commercial acidifier, and other acid-base pairs enter the calculation. Titration, by contrast, directly measures the carbonic acid present in the sample, and the blank reading subtracts the contribution of other acids.

Subtract the blank value from each reading

Other acids besides carbon dioxide can consume the reagent and skew the titration. Aerate 100 ml of your water for 15 minutes using an air pump and a diffuser, take a full reading on this water, then subtract that result from your actual measurement. Without this subtraction, your figure will be too high by the full blank value.

Amber water or peat-filtered water

The table shows a high CO2 level that the water does not actually contain, leading to an incorrect reduction of an injection that is beneficial to plants.

After a commercial acidifier

It lowers the pH without altering the carbon dioxide content, so the derived value rises whilst the actual concentration stays the same.

Clear, mineral water

The table and the titration converge. This is the case where the indirect method is valid.

In aquaponics

The foliage of your crops grows above the water and draws its carbon from the air. The test is most useful here to check that the CO2 released by the fish does not build up.

🐟 Too much CO2 for the fish

An excess of carbon dioxide does not change the appearance of the water, but it can be recognised by the behaviour of the fish: they come to the surface, open their gill covers wide, and become sluggish, exactly as if they were short of oxygen, even though the water may contain sufficient dissolved oxygen. When CO2 accumulates, the fish can no longer expel it through the gills and its blood becomes acidic. Vigorous aeration drives off the excess CO2, and the dissolved oxygen test allows you to distinguish between the two diagnoses.

Have a refill to hand while adjusting an injection system

Every reliable test requires two titrations, one for your water and one for the blank value, which means reagent 2 is consumed faster than you might expect. If you are adjusting a gas injection system, order the reagent refill at the same time as the test kit. That way you can take morning and evening readings throughout the adjustment period without interrupting the series just as the flow rate stabilises.

🌗 Two readings per day

In a planted volume, CO2 levels fall during the lighting period because photosynthesis consumes it, and rise at night when plants and bacteria release it without taking any in. Two readings on the same day, one in the morning before the lights come on and one in the late afternoon, therefore tell you far more than a weekly measurement taken at a random time, and they explain end-of-night die-offs in overdosed tanks. If you want the injection to follow this curve automatically, the CO2 control unit opens and closes the solenoid valve based on continuously measured pH, and this test then serves to verify the actual CO2 level.

Technical specifications

The essentials

Parameter measured: Dissolved CO2, in mg/LFree carbon dioxide dissolved in the water, not the carbonates measured by carbonate hardness
Method: Titration, counting the dropsNo shade to compare against a scale: you stop at the pink colour change, which must hold for 60 seconds
No reading: 1 drop = 2 mg/LSo, 10 drops of reagent 2 correspond to 20 mg/L of CO2
Target values: 15 to 30 mg/LSafe for fish. The ideal level is between 20 and 25 mg/L, up to 35 mg/L in a heavily planted tank
Covered environment: Freshwater onlyThis test does not cover fertilisers known as liquid carbon
Manufacturer's reference: JBL 2414000, PROAQUATEST rangeCode EAN 4014162241405

The procedure

Water volume: 20 ml, bubble-freePoured along the side of the container. A bubble carries carbon dioxide away and lowers the reading.
The steps: Rinse, fill, position, dose, titrate, countIn this order: 5 drops of reagent 1, then reagent 2 drop by drop until the colour changes
Waiting time: NoneThe colour is judged drop by drop; only the final pink must hold for 60 seconds.
Total duration: Approximately 3 minutesExcluding the blank reading, which requires 15 minutes of aeration
Reading: Top view, pink square on the colour chartThe two test tubes are compared from the top, one over the white square, the other over the pink
Correction required: Blank reading on aerated waterAerate 100 ml for 15 minutes, measure, then subtract that result from your reading

Security and parcels

Ranking: DangerBoth bottles are classified, and one of them is flammable
Main risks: Flammable, burns, eye damageH225 and H319 statements for reagent 1, H314 and H318 for reagent 2. Keep the bottles away from any naked flame
Precautions: Sink, well-ventilated room, out of reachChild-safety caps on both bottles
Parcel weight: 0.13 kgPackaging included

To budget for separately

A 20 ml syringe: For bubble-free samplingThe protocol requires 20 ml drawn off without any bubbles
Reagent refill: Reagents only, ref. 2414100The test tube and colour chart keep well

10 further specifications are available in the detailed view.

In the box

2 reagent bottles with child-resistant caps
1 graduated plastic test tube up to 20 ml
1 colour chart with 2 sections, one white and one pink
1 multilingual instruction leaflet, French included

Reference, EAN code, delivery contents, protocol, out-of-range readings, and hazard statements recorded from the manufacturer's data sheet and instructions on 05/09/2026. Fish tolerance ranges are drawn from aquaculture literature. For a titration-based test, the number of measurements per kit depends on the value being measured.

Measuring CO2, or calculating it?

Both kits include CO2 among their parameters, but they calculate it from a chart using pH and KH. This test measures it directly in the water. That is not the same information, and the two readings diverge as soon as the water is tannic or acidified.

The pH-KH chart gives false readings in tannin-stained water It assumes that all acidity comes from CO2. As soon as roots, peat or an acidifier are involved, it shows a CO2 level the water does not actually contain, whereas titration measures the dissolved gas directly.

	CO2 measured in water, by titration	CO2 derived from a pH and KH chart	Remains accurate in tannin-rich or acidified water	Parameters covered (parameters)	Drop tests included (tests)	pH, KH, nitrites and nitrates	GH, iron, phosphates, copper, silicates, oxygen	Reagent bottles (bottles)	Glass test tubes	Laboratory comparator block	Thermometer	Reagent refill sold separately	Price
Recommended CO2 test alone, direct measurementYour choice	Yes	No	Yes	1 parameters	1 tests	No	No	2 bottles	0	No	No	Yes	13,90 €
6-in-1 Aquarium Water Testing KitYour choice	No	Yes	No	6 parameters	5 tests	Yes	No	9 bottles	3	Yes	No	Yes	50,90 €
13-in-1 water testing kit for aquariumsYour choice	No	Yes	No	12 parameters	13 tests	Yes	Yes	23 bottles	12	Yes	Yes	Yes	119,90 €

CO2 test alone, direct measurementRecommended

How the CO2 is produced

CO2 measured in water, by titrationYes

CO2 derived from a pH and KH chartNo

Remains accurate in tannin-rich or acidified waterYes

What you're measuring

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

pH, KH, nitrites and nitratesNo

GH, iron, phosphates, copper, silicates, oxygenNo

Equipment supplied

Reagent bottles (bottles)2 bottles

Glass test tubes0

Laboratory comparator blockNo

ThermometerNo

Budget

Reagent refill sold separatelyYes

6-in-1 Aquarium Water Testing Kit

How the CO2 is produced

CO2 measured in water, by titrationNo

CO2 derived from a pH and KH chartYes

Remains accurate in tannin-rich or acidified waterNo

What you're measuring

Parameters covered (parameters)6 parameters

Drop tests included (tests)5 tests

pH, KH, nitrites and nitratesYes

GH, iron, phosphates, copper, silicates, oxygenNo

Equipment supplied

Reagent bottles (bottles)9 bottles

Glass test tubes3

Laboratory comparator blockYes

ThermometerNo

Budget

Reagent refill sold separatelyYes

13-in-1 water testing kit for aquariums

How the CO2 is produced

CO2 measured in water, by titrationNo

CO2 derived from a pH and KH chartYes

Remains accurate in tannin-rich or acidified waterNo

What you're measuring

Parameters covered (parameters)12 parameters

Drop tests included (tests)13 tests

pH, KH, nitrites and nitratesYes

GH, 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

Full titration protocol, 20 ml reference volume, stable pink endpoint rule of 60 seconds, and blank value determination.

PDF · 1,6 Mo

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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 dropper caps screwed on. The exact amount is shown in your basket.

Frequently asked questions

6 questions

6 key questions

Too much CO2 – what signs does it show on the fish?

What level should you aim for in a planted aquarium?

Does this test replace the pH and KH chart?

Do you really need to take a blank reading?

Take 100 ml of your water and aerate it for 15 minutes.
Carry out a full measurement on this aerated water, and note the number of drops.
Carry out your actual measurement on 20 ml of water from the tank.
Subtract the first result from the second.

Aerated water contains almost no CO2: whatever reagent it still consumes measures everything in your water that is not carbon dioxide gas. Without this subtraction, your figure will always come out too high.

My permanent indicator and this test give different readings — which should I trust?

Is this test necessary if I'm not injecting CO2?

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Dissolved CO2 water test in drops

The summary