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 water test in drops

Name: Potassium K water test in drops
Brand: JBL
SKU: J2413000
Price: 26.0 USD
Availability: InStock

$26.00

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 in too small a quantity for 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 it takes, the more potassium there is.
How long it takes: 4 minutes per measurement, shaking and resting included.
How to choose: potassium is not included in either of the two test kits, so this test is bought separately.
Precautions: both reagents are corrosive; the cloudy tube ends up down the sink and never goes into the fish's water.

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

This test measures dissolved potassium from 2 to 15 mg/L by turbidity, for about 25 measurements, and tells you when your aquaponic crops are running short of it. The reagent stays in the measuring tube, which you empty down the sink and never into the fish water. The K refill then reuses the same tubes and the same card.

In a planted aquarium, a complete liquid fertiliser covers the potassium requirement. In aquaponics, the situation is different: nitrogen arrives in abundance through the fish waste, whereas potassium hardly arrives at all, because the fish absorb it for their own needs and the feed is formulated very tightly. The better your system runs, the faster the plants grow, and the faster the potassium level falls. That explains a set-up with comfortable nitrate levels whose lettuces stagnate.

🔬 A turbidity reading

This test is not read on a colour scale. The second reagent makes the potassium 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 reading tube on 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, so that, counter-intuitively, the less you need to pour, the higher the level. Read under a bright, diffuse light, at the bottom of the meniscus. As there is no colour shade to judge, two people looking at the same tube read the same value.

Low value, plants that stall

This is the usual case in aquaponics. The input has to be made, and the measurement tells you 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 looks like

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 shows on the lower leaves, whose edges turn yellow and then necrotic while the young shoots keep their colour. Iron follows exactly the opposite logic, since it stays fixed where it was deposited and the deficiency appears on the leaves at the top. Looking first at where the yellowing starts therefore saves you from reaching for the wrong test, and the article on deficiencies in aquaponics covers the other patterns.

Cloudy water already skews the reading

The reading depends entirely on the opacity obtained 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 potassium and alkalinity in a single step

Nitrification uses up 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 double benefit saves you from buying two separate products. The carbonate hardness test gives you the second value, from the same sample.

🧪 How often to measure

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

🌊 Potassium among the other parameters

A potassium reading is never read in isolation, because three elements together govern leaf growth. Nitrogen, supplied free of charge by the fish, is rarely the limiting factor; potassium often is; iron becomes so as soon as the pH rises. A system whose nitrates are comfortable but whose plants stagnate all the same is often short of one of the other two, and two measurements will tell you which. The iron test complements this one when yellowing starts on the young leaves.

🥽 A caustic soda and a powder

Both reagents in this kit are corrosive: the liquid contains sodium hydroxide, and the powder that precipitates the potassium also carries the H314 hazard statement. Draw your 15 ml away from the tank, dose the powder with the scoop, out of any draught that would scatter it, and close each bottle straight away. Splashes on the skin or in the eye should be rinsed for a long time with clean water. The whitish suspension you end up with then goes down the sink: in an aquaponic system, it would put caustic soda back into the water where your fish live and where the roots sit. The test tube, the reading tube and the card are kept, and the K reagent refill replaces only the two bottles.

Technical specifications

Measuring potassium

Parameter measured: Dissolved potassium, in mg/LOf the three major nutrients, the one that fish waste supplies in the smallest amount
Reading range: 2.0 to 15 mg/LAbove 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 cloudiness obtained that gives the value
Reading the result: Disappearance of a printed crossViewed from above, in bright, diffuse light, at the bottom of the meniscus
Tests per kit: Around 2515 ml of water and one measuring spoon of powder per reading
Manufacturer's reference: JBL 2413000, PROAQUATEST range

The procedure

Sample volume: 15 ml per measureMeasured in the plastic graduated tube
The steps: Rinse, draw up, dose, shake, rest, pour, readIn that order. It is the final pour that gives the reading, not the colour obtained
Reagents to be measured out: 10 drops and 1 level scoopReagent 1 is liquid, reagent 2 is a powder. A heaped dose skews the value
Shaking and resting: 30 s, then 1 minThe precipitate must be uniform before pouring
Total time: About 4 minutesShaking and resting time included
Water types covered: Freshwater, pond, seawaterIn seawater, dilution to one thirtieth is compulsory

Hazard labelling

Signal word: DangerBoth the liquid and the powder carry this warning
Pictogram: Corrosive, GHS05The same on both bottles
Hazard statements: H314Skin burns and eye damage, a statement carried by both bottles; declared component of reagent 1: sodium hydroxide
Precautionary statements: P101, P102, P303+P361+P353, P305+P351+P338, P501Label to show the doctor, keep out of reach of children, rinse skin and eyes with water, dispose of bottles according to local rules
Precautions: Sink, ventilated room, out of reachThe powder of reagent 2 should be dosed away from draughts, as it disperses easily
After reading: Tube emptied down the sinkThe white suspension must never go back into the tank

To allow for in addition

Enough to correct: Potassium bicarbonateIt raises the potassium content and the carbonate hardness in a single addition
Reagent refills: Reagents only, ref. 2413100The two tubes, the syringe, the measuring scoop and the reading card are all kept

12 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 sampling tube, graduated in mg/L of potassium
1 graduated sampling syringe
1 double measuring spoon, one wide end and one narrow
1 reading card bearing the reference cross, with multilingual instructions, French included

Manufacturer's values (instructions for use, reading card and safety data sheets for the two reagents).

Manufacturer and product safety

Manufacturer: JBL GmbH & Co. KG
Postal address: Dieselstraße 3, 67141 Neuhofen, Allemagne
Contact: info@jbl.de
Website: www.jbl.de

Information provided under Regulation (EU) 2023/988 on general product safety.

What this test adds to any kit

Neither of the two kits measures potassium. This test sits alongside them rather than replacing them.

Fish waste provides almost no potassium Nitrogen and phosphorus arrive with the feed; potassium, very little. It is the first element to run short as soon as a plant starts to fruit.

	Potassium measured	Parameters covered	Drop test kits included	pH, KH, nitrite and nitrate	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	1	No	No	2 bottles	1	No	No	Yes	$26.00
6-in-1 Aquarium Water Testing KitYour choice	No	6	5	Yes	No	9 bottles	3	Yes	No	Yes	$58.00
JBL ProAquaTest Lab water analysis case, 13 tests for aquarium and pondYour choice	No	12	13	Yes	Yes	23 bottles	12	Yes	Yes	Yes	$114.00

Potassium-only testRecommended

Potassium

Potassium measuredYes

Parameters covered1

Drop test kits included1

What each kit covers

pH, KH, nitrite and nitrateNo

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

Potassium measuredNo

Parameters covered6

Drop test kits included5

What each kit covers

pH, KH, nitrite and nitrateYes

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

JBL ProAquaTest Lab water analysis case, 13 tests for aquarium and pond

Potassium

Potassium measuredNo

Parameters covered12

Drop test kits included13

What each kit covers

pH, KH, nitrite and nitrateYes

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 and equipment of the two cases taken from their data sheets.

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

0.13 kg parcel, dispatched within 48 hours by Colissimo or Chronopost home delivery, as you choose at checkout. Reagents are sent in their original box, tubes and accessories included. The exact amount is shown in your basket. Once unsealed, this product cannot be returned under the right of withdrawal (article L221-28, 5° of the French Consumer Code), for reasons of hygiene and health protection.

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 recognise a deficiency in my plants?

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

Potassium moves freely in the sap, whereas iron barely moves at all. When potassium runs short, it is remobilised from the old leaves to the new shoots: the symptom therefore appears at the bottom, along the leaf edge. Iron, by contrast, stays where it was deposited, so its deficiency appears at the top, on the new leaves, between the veins. Look first at where the yellowing begins; that saves you from testing for 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-test 24 hours after the last dose, never straight after.

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

The bicarbonate dissolves in the system water; the test reagents, however, never go into the fish water, and the cloudy tube is emptied down the sink.

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 water test in drops

The summary