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

Nutrient deficiency diagnostic chart: mobile elements such as magnesium cause yellowing on the older lower leaves, while poorly mobile elements such as iron cause yellowing on the younger upper leaves

Magnesium water test in drops, freshwater

Name: Magnesium water test in drops, freshwater
Brand: JBL
Price: 21.0 USD
Availability: InStock

$21.00

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.

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The summary

What you are measuring: dissolved magnesium, from 0 to 10 mg/L, across eight colour fields, in freshwater only.
Why measure it: aim for 5 to 10 mg/L for healthy growth. Tap water is often deficient, even when it is very hard.
What you receive: 3 reagent bottles, 2 screw-cap glass test tubes, a sampling syringe, a comparator block, and a colour chart.
How long it takes: three minutes, and the colour remains readable for fifteen minutes — no longer.
How to choose: a total hardness test combines calcium and magnesium without distinguishing between them.
What's sent to you: a parcel weighing 0.12 kg, with reagents in their original packaging.

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

A leaf yellowing between the veins whilst the veins themselves remain distinctly green almost always prompts a purchase of iron. That instinct is right only half the time, because magnesium produces exactly the same picture, to the point where systems receive weeks of chelate without anything turning green again. This test distinguishes between the two in three minutes, over a range of 0 to 10 mg/L and for around sixty measurements.

🍂 Lower leaves, upper leaves

Everything comes down to how mobile the element is within the plant. Magnesium moves through the sap and is remobilised from older tissue to growing organs as soon as the supply becomes insufficient, which means the lower part of the plant yellows first. Iron, on the other hand, barely moves at all: once deposited in a leaf, it stays there, and the deficiency shows up on new leaves, at the top. Start by looking at where the yellowing begins on your plants, and you'll know which of the two test kits to reach for before you've even taken a single sample.

🥬 The magnesium in chlorophyll

Every chlorophyll molecule is built around a single magnesium atom, and no other mineral can take its place. A plant that runs short therefore stops producing the pigment that captures light, which explains why discolouration always precedes stunted growth by several weeks. You can see the deficiency coming long before it costs you a harvest, provided you know that this particular yellowing can have two distinct causes.

Colour loss begins between the veins because that is where the tissue responsible for photosynthesis is concentrated, while the veins themselves, made up of conducting vessels, retain their colour longer. This pattern of green veins against a pale background is the shared signature of both magnesium and iron deficiency, which is precisely why the eye alone cannot distinguish between them. Our articles on deficiencies in aquaponics cover the related charts, including nitrogen, potassium, and iron.

Remineralising without forgetting magnesium

Osmosis water remineralised with calcium alone will show a perfectly acceptable GH and a magnesium level of zero, this is the most common pitfall in soilless growing. Epsom salt corrects precisely this imbalance, as it supplies magnesium and sulphate with no carbonate whatsoever, leaving your buffering capacity untouched. A carbonate-based corrector, by contrast, would raise both the KH and pH at the same time. Factor it into your top-up routine rather than treating it as an emergency fix.

Epsom salt is dosed at a tenth of what most people think

The scale surprises you the first time: 1 gram in 10 litres raises magnesium by approximately 10 mg/L, that is, the entire range of the test chart in one go. Dose small, allow a full filtration cycle to complete, and only retest afterwards. An excess can only be corrected with a partial water change, which takes far more time than the half-day's wait you were trying to avoid.

💧 What GH combines

Magnesium ions and calcium ions together make up total hardness, which the GH test measures as a single figure without ever distinguishing between them. Very hard tap water can therefore contain plenty of calcium yet far too little magnesium for aquatic plants, and nothing in the GH reading will reveal this. That is the purpose of this kit: it isolates the one fraction on which chlorophyll depends. If you were to track only one value in remineralised water, this would be the one to choose rather than GH.

🧪 When testing is worth the effort

Yellowing starts from the bottom

The older leaves pale between green veins while new growth looks healthy. That is the classic picture of magnesium deficiency.

Yellowing starts at the top

New leaves emerge pale while the lower growth stays green. Reach for the iron test, not this one.

Below 5 mg/L, plants perform adequately

This is not an emergency. Monitor the leaf before making any corrections.

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 shows mg/l (ppm): both units are equivalent in freshwater.
Measurement range: 0 to 10 mg/L, plus a field for values beyond thatThe scale narrows at the lower end, where the decision really matters
The colour chart fields: 0, 1, 2, 3, 6, 8, 10 and over 10Eight values, the last field being shown as more than 10 mg/L
Target values: 5 to 10 mg/LOptimum growth range for aquatic plants
Tests per kit: Around 60The highest number of tests in the entire range
Manufacturer reference: JBL 2414200, PROAQUATEST rangeCode EAN 4014162241429

The procedure

Water volume: 2 x 10 mlOne treated sample and one untreated control sample, side by side in the comparator block
The steps: Rinse, fill, dose 1, dose 2, dose 3, readWith a stir between each addition of reagent
Drops per reagent: 7, then 3, then 2Three different dosages: this is the test in the range where the order and the count are most easily confused.
Waiting time: 1 minuteAnd the colour remains stable for only fifteen minutes: it is the only test in the range with a closed reading window
Total duration: Approximately 3 minutesHandling included
Reading: Colour chart, in natural daylightThe block slides along the colour chart until the two shades match

Security and parcels

Ranking: DangerThe three bottles are ranked
Main risks: Burns, eye injuriesH314 and H318 statements
Precautions: Sink, ventilated room, out of reachChild-safety caps on all three bottles
Covered environment: Freshwater onlySeawater reads around a thousand mg/L, well beyond this scale

To budget for separately

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

10 further specifications are available in the detailed view.

In the box

3 reagent bottles with child-safety 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 manual, 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 0 to 10 mg/L range 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 combines calcium and magnesium without separating them. Here is what each column actually tells you when your question is about magnesium.

GH combines calcium and magnesium without distinguishing between them Two water samples both showing 8 °dGH can contain one with three times more magnesium than the other. As soon as you are dosing a salt yourself, the overall figure is no longer enough.

	Parameters covered (parameters)	Magnesium measured separately, in mg/L	Calcium and magnesium counted together	The nitrogen cycle in full	Reagent bottles (bottles)	Glass test tubes	Laboratory comparator block	Colour chart	Measurement duration (minutes)	Reagent refill sold separately	Price
Recommended Magnesium test aloneYour choice	1 parameters	Yes	No	No	3 bottles	2	Yes	Yes	3 minutes	Yes	$21.00
Total Hardness TestYour choice	1 parameters	No	Yes	No	1 bottles	0	No	No	2 minutes	Yes	$11.00
14-in-1 water analysis kit for aquariumYour choice	14 parameters	No	Yes	Yes	23 bottles	12	Yes	Yes	30 minutes	Yes	$104.00

Magnesium test aloneRecommended

What the reading tells you

Parameters covered (parameters)1 parameters

Magnesium measured separately, in mg/LYes

Calcium and magnesium counted togetherNo

The nitrogen cycle in fullNo

Equipment supplied

Reagent bottles (bottles)3 bottles

Glass test tubes2

Laboratory comparator blockYes

Colour chartYes

Installation

Measurement duration (minutes)3 minutes

Reagent refill sold separatelyYes

Total Hardness Test

What the reading tells you

Parameters covered (parameters)1 parameters

Magnesium measured separately, in mg/LNo

Calcium and magnesium counted togetherYes

The nitrogen cycle in fullNo

Equipment supplied

Reagent bottles (bottles)1 bottles

Glass test tubes0

Laboratory comparator blockNo

Colour chartNo

Installation

Measurement duration (minutes)2 minutes

Reagent refill sold separatelyYes

14-in-1 water analysis kit for aquarium

What the reading tells you

Parameters covered (parameters)14 parameters

Magnesium measured separately, in mg/LNo

Calcium and magnesium counted togetherYes

The nitrogen cycle in fullYes

Equipment supplied

Reagent bottles (bottles)23 bottles

Glass test tubes12

Laboratory comparator blockYes

Colour chartYes

Installation

Measurement duration (minutes)30 minutes

Reagent refill sold separatelyYes

Parameters covered, equipment supplied and kit contents recorded from all three product pages on 05/09/2026. The 14-in-1 kit brings together 13 drop tests plus CO2 read from a colour 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 to a collection point or via Colissimo home delivery, your choice at checkout. The reagents ship upright in their original box. The exact cost is shown in the 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 deficiency?

Three deficiencies produce yellowing foliage, and the location of the symptom tells them apart.

Where it starts	Appearance	The element
New leaves, at the top	Pale blade, green veins	Iron
Old leaves, at the bottom	Yellowing between the veins	Magnesium
Old leaves, at the bottom	Edges yellowing then drying out	Potassium

Iron does not move within the plant; magnesium and potassium do. That is why iron shows up at the top and the other two at the bottom.

What magnesium level should you aim for?

My GH is correct — can I still be short of magnesium?

Yes, and it is a common occurrence. Total hardness adds calcium and magnesium together without distinguishing between them: water that is very high in calcium but low in magnesium will show a high GH while leaving your plants without magnesium. The clearest example is reverse osmosis water remineralised with a calcium salt alone, which produces a perfect GH reading and zero magnesium.

How do you raise magnesium without changing the rest of the water chemistry?

Why does the colour only remain legible for fifteen minutes?

Is an excess of magnesium harmful?

What calcium-to-magnesium ratio should be maintained?

Why is this test restricted to freshwater?

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Magnesium water test in drops, freshwater

The summary