Freshwater magnesium water test box, 10.6 by 17.5 cm format, labelled 60 measurements

Contents of the magnesium water test kit: comparator block, two glass test tubes, syringe, three reagent bottles and colour chart from 0 to over 10 mg per litre.

Reading the magnesium test: both test tubes placed in the comparator block, slid over the colour chart

The three reagent bottles of the freshwater magnesium test, numbered 1, 2 and 3

Magnesium water test in drops, freshwater

Name: Magnesium water test in drops, freshwater
Brand: Hardy Dewerse
SKU: J2414200
Price: 23.0 USD
Availability: InStock

$23.00

$7.66 × 3 with no fees Klarna.

This drop-based test measures magnesium from 0 to 10 mg/L in freshwater, whether in a planted aquarium or in aquaponics. Magnesium is the central atom of chlorophyll, and a plant deficient in it will yellow between the veins.

An iron deficiency produces the same yellowing, but it starts with the young leaves, whereas a magnesium deficiency starts with the lower leaves.

Ref. J2414200·Water test

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The summary

What you measure: dissolved magnesium, from 0 to 10 mg/L, across eight colour fields, in fresh water only.
Why measure it: aim for 5 to 10 mg/L for good growth. Very hard water may nonetheless contain little of it.
What you receive: 3 bottles of reagents, 2 glass test tubes with screw caps, a syringe, a comparator block and a colour chart.
How long it takes: 3 minutes, and the colour stays readable for 15 minutes, no longer.
How to choose: a total hardness test adds calcium and magnesium together without separating them.
What is sent to you: a 0.17 kg parcel, reagents in their original box.

Drop test for magnesium from 0 to 10 mg/L, in fresh water, to distinguish between iron and magnesium deficiency

A leaf that yellows between veins that have stayed perfectly green often suggests an iron deficiency first. Magnesium produces the same picture, however, so a chelate supplement can go on for weeks without the foliage turning green again. This test tells the two apart in 3 minutes, over a range of 0 to 10 mg/L and for around sixty measurements.

🍂 Lower leaves, upper leaves

The difference comes down to how mobile the element is within the plant. Magnesium travels in the sap and is remobilised from older tissues to growing organs as soon as the supply becomes insufficient, so the base of the plant turns pale first. Iron, on the other hand, hardly moves at all: once deposited in a leaf, it stays there, and the deficiency shows on the new leaves, at the top. So start by looking at where the yellowing begins on your plants, and you will know which of the two kits to take out before you have even drawn a single drop.

🥬 The magnesium in chlorophyll

Every chlorophyll molecule is built around a single magnesium atom, and no other mineral can fill that role. A plant that lacks it produces less of this light-capturing pigment, so its leaves turn pale and its growth stalls. Paleness is therefore the first sign to watch for on your plants, bearing in mind that this yellowing can have two causes.

The loss of colour begins between the veins because that is where the tissue responsible for photosynthesis is concentrated, while the veins themselves, made of conducting vessels, keep their colour for longer. This green network pattern on a pale background is the shared signature of magnesium and iron, which is exactly why the eye alone cannot tell them apart. Our articles on deficiencies in aquaponics cover the neighbouring tables, including nitrogen, potassium and iron.

Remineralise without forgetting magnesium

Osmosis water remineralised with calcium alone shows a perfectly good GH and a magnesium content of zero, which is a common imbalance in remineralised water. Epsom salt corrects this specific imbalance, since it supplies magnesium and sulphate with no carbonate at all and so leaves your buffering capacity intact. A carbonate-based corrector, by contrast, would raise the KH and the pH along the way.

Dose the Epsom salt to the gram

The order of magnitude is surprising the first time: 1 gram in 10 litres raises magnesium by about 10 mg/L, which is the whole range of the colour chart in one go. Dose small, let the water circulate through one full filtration cycle, and only then retest. An excess can only be corrected with a partial water change, which costs far more time than the half-day of waiting you saved.

💧 What the GH blends

Magnesium ions and calcium ions together make up the total hardness, which the GH test counts as a single block without ever telling them apart. Very hard tap water can therefore contain plenty of calcium and too little magnesium for aquatic plants, and nothing in the GH reveals it. That is the purpose of this kit: it isolates the one fraction on which chlorophyll depends. On remineralised water, measure both: the GH gives the total, this test gives the magnesium share.

🧪 When testing is worth it

Yellowing starts at the bottom

The old leaves turn pale between green veins, while the new shoots are fine. This is the magnesium table.

Yellowing starts from the top

New leaves come out pale while the lower ones stay green. Reach for the iron test, not this one.

Under 5 mg/L, plants do reasonably well

It is not an emergency. Keep an eye on the leaf before correcting anything.

After remineralisation

A salt that supplies only calcium leaves magnesium at zero. Measure from the very first fill.

Technical specifications

The essentials

Parameter measured: Dissolved magnesium, in mg/LThe colour chart is marked mg/l (ppm): the two units are equivalent in fresh water
Measurement range: 0 to 10 mg/L, plus a range beyond thatThe steps get closer together at the low end of the scale, where the decision is made
The colour chart fields: 0, 1, 2, 3, 6, 8, 10 and over 10Eight values, the last field being recorded as more than 10 mg/L
Target values: 5 to 10 mg/LRange for good growth of aquatic plants
Tests per kit: Approximately 60
Manufacturer's reference: JBL 2414200, PROAQUATEST range

The procedure

Water volume: 2 times 10 mlOne treated sample and one untreated blank, side by side in the comparator block
The steps: Rinse, fill, dose 1, dose 2, dose 3, readWith a mix between each addition of reagent
Drops per reagent: 7, then 3, then 2Three different measurements to note down before you begin
Waiting time: 1 minuteThe colour then stays stable for 15 minutes: take your reading within that window
Total duration: Approximately 3 minutesHandling included
Reading: Colour chart, in daylightThe block slides along the colour chart until the two shades match

Security and parcels

Ranking: DangerOnly reagent 2 is classified; it contains sodium hydroxide
Main risks: Burns, eye injuriesH314 and H318 statements
Precautions: Sink, well-ventilated room, out of reachChild-resistant caps on all three bottles
Covered environment: Freshwater onlySeawater measures around a thousand mg/L, far beyond this scale

To budget for separately

What to correct with: Epsom salt, magnesium sulphate1 gram in 10 litres raises the magnesium content by about 10 mg/L: dose sparingly
Reagent refill: Reagents only, ref. 2414300The glassware, comparator block and colour chart can be kept

10 further specifications are available in the detailed view.

In the box

3 reagent bottles with child-resistant caps
2 glass test tubes with screw caps
1 graduated sampling syringe
1 laboratory comparator block, in grey
1 colour chart with 8 colour fields, from 0 to over 10 mg/L
1 multilingual instruction leaflet, French included

Reference, EAN code, number of tests, kit contents, six-step protocol, target value and hazard statements taken from the manufacturer's data sheet and instructions on 05/09/2026. The range of 0 to 10 mg/L and the detail of the eight fields are taken from a photograph of the colour chart supplied.

Magnesium, or total hardness?

A total hardness test adds calcium and magnesium together without separating them. Here is what each column really tells you when your question is about magnesium.

GH adds up calcium and magnesium without separating them Two waters showing 8 °dGH can contain, in one case, three times as much magnesium as in the other. As long as you are dosing a salt yourself, the overall figure is no longer enough.

	Parameters covered (parameters)	Magnesium dosed separately, in mg/L	Calcium and magnesium counted together	The complete nitrogen cycle	Reagent bottles (bottles)	Glass test tubes	Laboratory comparator block	Colour chart	Duration of a measurement (minutes)	Reagent refill sold separately	Price
Recommended Magnesium-only testYour choice	1 parameters	Yes	No	No	3 bottles	2	Yes	Yes	3 minutes	Yes	$23.00
GH total hardness water test in dropsYour choice	1 parameters	No	Yes	No	1 bottles	0	No	No	2 minutes	Yes	$12.00
13-in-1 water testing kit for aquariumsYour choice	12 parameters	No	Yes	Yes	23 bottles	12	Yes	Yes	30 minutes	Yes	$138.00

Magnesium-only testRecommended

What testing teaches you

Parameters covered (parameters)1 parameters

Magnesium dosed separately, in mg/LYes

Calcium and magnesium counted togetherNo

The complete nitrogen cycleNo

Equipment supplied

Reagent bottles (bottles)3 bottles

Glass test tubes2

Laboratory comparator blockYes

Colour chartYes

How to use

Duration of a measurement (minutes)3 minutes

Reagent refill sold separatelyYes

GH total hardness water test in drops

What testing teaches you

Parameters covered (parameters)1 parameters

Magnesium dosed separately, in mg/LNo

Calcium and magnesium counted togetherYes

The complete nitrogen cycleNo

Equipment supplied

Reagent bottles (bottles)1 bottles

Glass test tubes0

Laboratory comparator blockNo

Colour chartNo

How to use

Duration of a measurement (minutes)2 minutes

Reagent refill sold separatelyYes

13-in-1 water testing kit for aquariums

What testing teaches you

Parameters covered (parameters)12 parameters

Magnesium dosed separately, in mg/LNo

Calcium and magnesium counted togetherYes

The complete nitrogen cycleYes

Equipment supplied

Reagent bottles (bottles)23 bottles

Glass test tubes12

Laboratory comparator blockYes

Colour chartYes

How to use

Duration of a measurement (minutes)30 minutes

Reagent refill sold separatelyYes

Parameters covered, equipment supplied and kit contents taken from the three data sheets on 05/09/2026. The 13-in-1 kit combines 13 drop tests, three of them for pH, plus CO2 read from a chart.

Documents & manuals

Official manufacturer's instructions, in French

Six-step protocol, volumes and number of drops reagent by reagent, target value for plant growth, and what to do when the result is too low or too high.

PDF · 1,6 Mo

Download

Estimate your
delivery time

0.12 kg parcel, dispatched within 48 hours by Colissimo or Chronopost to your home, as you choose at checkout. The reagents are sent in their original box, upright. The exact cost is shown in your basket.

Frequently asked questions

8 questions

6 key questions, 8 in the detailed view

My leaves are yellowing between the veins, is it iron or magnesium?

Three deficiencies produce yellow foliage, and the location of the symptom tells all three apart.

Where it starts	Appearance	The element
New leaves, at the top	Pale blade, green veins	Iron
Old leaves, at the bottom	Paleness between the veins	Magnesium
Old leaves, at the bottom	Edge that yellows then dries	Potassium

Iron does not move around within the plant, whereas magnesium and potassium do. That is why iron marks the top and the other two the bottom.

What magnesium level should you aim for?

My GH is fine, can I still be short of magnesium?

How can I raise magnesium without changing the rest of the water?

Why does the colour stay readable for only 15 minutes?

Is an excess of magnesium dangerous?

What calcium to magnesium ratio should be maintained?

Why is this test only for freshwater?

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Magnesium water test in drops, freshwater

The summary