Front-facing view of the potassium K water test box on a white background, labelled 25 tests

The two reagent bottles for the potassium K test — the liquid and the powder — on a white background

Diagram of potassium deficiency: the edges of older lower leaves turn brown while the new growth remains green

Diagram of turbidity reading: three graduated tubes filled to different levels for 2, 8 and 15 mg/L of potassium

Potassium K drop water test

Name: Potassium K drop water test
Brand: Hardy Dewerse
SKU: J2413000
Price: 22.9 EUR
Availability: InStock

22,90 €

7,63 € × 3 with no fees Klarna.

Fish feed provides little potassium, whereas plants consume a lot of it: in aquaponics, its level falls over successive harvests with no sign in the water.

This test gives the value in mg/L, from 2.0 to 15, without a colour chart: you pour in the cloudy water until a printed cross disappears, and read the graduation reached.

Ref. J2413000·Water test

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The summary

Why this test exists: potassium is, along with calcium and iron, one of the elements that fish feed supplies too little of for the crops.
What you receive: 2 bottles of reagents, a graduated measuring tube, a glass reading tube, a syringe, a double measuring spoon and a reading card.
How it is read: by turbidity. You pour until a printed cross is hidden, then read the graduation. The less you need, the more potassium there is.
How long it takes: 4 minutes per measurement, shaking and settling included.
How to choose: potassium is not included in either of the two test cases, so this test is bought separately.
What is sent to you: a 0.13 kg parcel, with the reagents in their original box.

Dissolved potassium test from 2.0 to 15 mg/L, read by turbidity, for aquaponics and planted tanks

In a planted aquarium, a complete liquid fertiliser covers the need for potassium. In aquaponics, the situation is different: nitrogen arrives on its own and in abundance through the fish waste, whereas potassium hardly arrives at all: the fish absorb it for their own needs and the feed is formulated as tightly as possible. The better your system runs, the more the plants grow, and the faster the potassium level falls. This is what explains an installation with comfortable nitrate levels whose lettuces are stalling.

🔬 A turbidity reading

This test is not read from a colour chart. The second reagent causes the potassium to precipitate as a whitish suspension, denser the more potassium the water contains, and it is this cloudiness that serves as the measurement. You place the glass test tube over the printed cross supplied with the kit, then pour in the clouded water until the cross, viewed from above, disappears. The height reached gives the value, which has a puzzling consequence the first time you use it: the less you need to pour, the higher the level. Read it in bright, diffuse light, at the bottom edge of the meniscus. As there are no colour shades to judge, two people looking at the same tube will read the same value.

Low reading, plants stalling

This is the usual case in aquaponics. The input has to be made, and the measurement tells you by how much.

Low value, healthy plants

No correction is needed. In a dimly lit tank, a low value is not a deficiency.

High reading with no input

Look at your top-up water, or at a fertiliser that is already rich in potassium.

Above 15 mg/L

Dilute 10 ml in 300 ml of distilled water, then multiply the result by 30.

🥬 What the deficiency shows

Potassium moves freely in the sap, so it is remobilised from older tissues to growing organs as soon as the supply becomes insufficient. The deficiency therefore appears on the lower leaves, whose edges yellow and then die back while the young shoots keep their colour. Iron follows exactly the opposite logic, since it stays fixed where it was deposited and the deficiency shows on the leaves at the top. Looking first at where the yellowing begins therefore avoids reaching for the wrong test, and our article on deficiencies in aquaponics covers the other patterns.

Water that is already cloudy skews the reading

The reading relies entirely on the cloudiness produced after the reaction, which assumes the water is clear to begin with. Water loaded with particles, bacterial haze or fine grow medium will give a value that is too high, with no sign to warn you. Let your sample settle, or filter it, before you begin.

Correct the potassium and the alkalinity in one go

Nitrification consumes alkalinity, so an aquaponic system loses its buffering capacity at the same time as it loses its potassium. Both drops can be corrected together with potassium bicarbonate: the hydrogen carbonate ion raises the KH while the potassium ion feeds the crops. This dual action saves you from buying two separate products. The carbonate hardness test gives you the second value, from the same sample.

🧪 How often to test

The potassium level does not vary from one day to the next the way nitrite does during cycling: it declines slowly, at the pace of plant growth, then rises all at once after a feed. A monthly reading is therefore enough on a stable system, provided you repeat it as soon as a symptom appears on the lower part of the plants or a large harvest has just been taken. Take another reading a few days after a feed too, since it is the difference between your two readings that shows what your crops are actually consuming. The 25 tests in the kit therefore cover two good seasons of monitoring.

🌊 Potassium among the other parameters

A potassium reading should never be interpreted on its own, because three elements together govern leaf growth. Nitrogen, supplied free of charge by the fish, is rarely the limiting factor; potassium often is; iron is as soon as the pH rises. An installation whose nitrates are comfortable but whose plants stall all the same has therefore run short of one of the other two, and two measurements are enough to tell which. The iron test complements this one when yellowing starts with the young leaves.

Technical specifications

The essentials

Parameter measured: Dissolved potassium, in mg/LOf the three major nutrients, the one that fish waste supplies least
Reading range: 2.0 to 15 mg/LBeyond that, dilute 10 ml in 300 ml of distilled water and multiply by 30
Method: Turbidity, not colorimetryPotassium precipitates as a whitish suspension: it is the resulting cloudiness that gives the value
Reading the result: Disappearance of a printed crossViewed from above, in bright, diffuse light, at the bottom edge of the meniscus
Tests per kit: Approximately 2515 ml of water and one scoop of powder per measurement
Manufacturer's reference: JBL 2413000, PROAQUATEST range

The procedure

Sample volume: 15 ml per measurementMeasured in the plastic graduated tube
The steps: Rinse, sample, dose, shake, wait, pour, readIn that order. It is the final dose that gives the reading, not the colour obtained
Reagents to dose: 10 drops and 1 level scoopReagent 1 is liquid, reagent 2 is a powder. A heaped dose skews the value
Stirring and resting: 30 s, then 1 minThe precipitate must be uniform before pouring
Total duration: Approximately 4 minutesShaking and resting time included
Environments covered: Freshwater, pond, seawaterIn seawater, dilution to one thirtieth is mandatory

Security and parcels

Ranking: DangerBoth bottles are sorted
Main risks: Burns, eye injuriesH314 and H318 statements
Precautions: Sink, well-ventilated room, out of reachThe powder of reagent 2 must be measured out away from any draught, as it disperses easily
Parcel weight: 0.13 kgPackaging included

To budget for separately

What to correct with: Potassium bicarbonateIt raises the potassium content and the carbonate hardness in a single dose
Reagent refill: Reagents only, ref. 2413100The two tubes, the syringe, the measuring scoop and the reading card are stored

10 further specifications are available in the detailed view.

In the box

2 reagent bottles, one liquid and one powder
1 plastic measuring tube, graduated from 5 to 20 ml
1 glass test tube, graduated in mg/L of potassium
1 graduated sampling syringe
1 double measuring scoop, one wide end and one narrow end
1 reading card with the reference cross, with multilingual instructions, French included

Reference, EAN code, number of measures, kit contents, seven-step protocol, dilution rule and hazard statements taken from the manufacturer's data sheet and instructions on 05/09/2026. Range of 2.0 to 15 mg/L taken from the manufacturer's documentation. Values recommended in aquaponics vary by source, from 10 to over 100 mg/L.

What this test adds to any kit

Potassium is the only one of the three major nutrients that the two cases do not measure. This test sits alongside them rather than replacing them.

Fish waste provides almost no potassium Nitrogen and phosphorus come in with the feed; potassium does not. It is the first element to run short once a plant starts fruiting.

	Measured potassium	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 Potassium-only testYour choice	Yes	1 parameters	1 tests	No	No	2 bottles	1	No	No	Yes	22,90 €
6-in-1 Aquarium Water Testing KitYour choice	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	12 parameters	13 tests	Yes	Yes	23 bottles	12	Yes	Yes	Yes	119,90 €

Potassium-only testRecommended

Potassium

Measured potassiumYes

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

What each kit covers

pH, KH, nitrites and nitratesNo

GH, iron, phosphates, copper, silicates, oxygenNo

Equipment supplied

Reagent bottles (bottles)2 bottles

Glass test tubes1

Laboratory comparator blockNo

ThermometerNo

Budget

Reagent refill sold separatelyYes

6-in-1 Aquarium Water Testing Kit

Potassium

Measured potassiumNo

Parameters covered (parameters)6 parameters

Drop tests included (tests)5 tests

What each kit covers

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

Potassium

Measured potassiumNo

Parameters covered (parameters)12 parameters

Drop tests included (tests)13 tests

What each kit covers

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 seven-step protocol, 15 ml volume, reading rule at the lower meniscus, one-in-thirty dilution and recommended potassium values.

PDF · 1,5 Mo

Download

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 tubes and accessories included. The exact amount is shown in your basket.

Frequently asked questions

8 questions

6 key questions, 8 in the detailed view

What potassium level should you aim for?

Why is potassium so often lacking in aquaponics?

How do I spot a deficiency in my plants?

Edges of older leaves or young shoots: potassium or iron?

Potassium moves around the plant and iron does not. A plant short of potassium pulls it from its old leaves to feed its new shoots, so the symptom appears at the bottom, along the leaf edge. A plant short of iron cannot redistribute anything, so the symptom appears at the top, on the new leaves, between the veins. First look at where the yellowing starts; that saves you from measuring both.

How can you add potassium without upsetting the rest of the water?

Measure your potassium and your carbonate hardness on the same day.
If both are low, use potassium bicarbonate: it corrects both.
Add it in small doses, spread over several days.
Re-measure 24 hours after the last dose, never straight afterwards.

Potassium sulphate supplies potassium without affecting the buffer, so it is the right choice when your carbonate hardness is already correct.

How do I measure beyond 15 mg/L?

Can an excess of potassium cause a problem?

Why only 25 measurements?

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Potassium K drop water test

The summary