Box for the KH carbonate hardness water test, 10.6 by 17.5 cm, reagent and test tube inside

Front view of the KH carbonate hardness test box on a white background

Contents of the KH carbonate hardness test: the reagent bottle and the graduated plastic test tube from 5 to 20 ml

Carbonate hardness measurement in progress: the reagent being added drop by drop to the test tube held by hand

Carbonates hardness test reading chart: one drop equals 1 degree KH, recommended ranges from 3 to 20 degrees depending on the tank, and alkalinity consumed by nitrification

KH water test, carbonate hardness, drop-based

Name: KH water test, carbonate hardness, drop-based
Brand: JBL
Price: 12.0 USD
Availability: InStock

$12.00

Carbonate hardness is the buffering capacity of your water, its ability to neutralise acidification without the pH fluctuating. This test measures it in aquariums, ponds, and aquaponics systems.

The measurement is done by titration: you count the drops until the colour changes from blue to orange-yellow, and each drop equals 1 °dKH.

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The summary

What you receive: 1 bottle of KH reagent, 1 plastic test tube graduated from 5 to 20 ml and multilingual instructions.
What you are measuring: the carbonates that stabilise your pH, in freshwater, in ponds, in seawater and in aquaponics.
How to read the result: by titration, one drop equals 1 °dKH. No colour chart, no shade to interpret, just a simple count.
Test duration: 2 minutes, with no waiting at all. The colour change is immediate.
How to choose: KH is also included in the two test kits, which add pH, nitrite and nitrate.
A reagent classified as Danger: flammable and corrosive, its drops are counted over a sink, never into the fish's water.

Drop titration test, one drop equals 1 °dKH, for aquariums, ponds and aquaponics

This test measures carbonate hardness, the buffering capacity that stabilises the pH, by titration: one drop equals 1 °dKH, in 2 minutes. In aquaponics, nitrification uses it up continuously, hence a weekly check. The corrosive reagent stays in the test tube, never in the fish water; the KH refill reuses the same kit.

Carbonate hardness, written KH, measures the dissolved carbonates and bicarbonates, that is, the buffering capacity of the water: they neutralise the acids produced in the tank and keep the pH stable. A pH that swings sharply from one day to the next therefore almost always points to a KH that is too low, because once these carbonates are used up, every addition of acid makes the pH drop with nothing to cushion it.

🧱 The 5 °dKH floor

Carbonate hardness should not drop below 5 °dKH, whether in a garden pond or in a tank where you inject CO2, and most freshwater fish and plants thrive between 5 and 16 °dKH. Below that, there are not enough carbonates left to absorb the acids continuously produced by your fish's respiration and by nitrification, and the pH can crash overnight. A sudden drop like this is far more dangerous for fish than a pH that is persistently off target, which is why you should measure KH at the same time as pH, never separately.

🌿 KH in aquaponics

Nitrification is an acidifying reaction: the bacteria that convert ammonium into nitrite and then into nitrate release protons, which are neutralised by the alkalinity of your water. Nitrifying 1 mg/L of ammonia nitrogen consumes about 0.4 dKH, so a system that converts 5 mg/L of nitrogen loses about 2 dKH. Your setup therefore loses alkalinity week after week, and nothing replenishes it if your top-up water is soft. Potassium bicarbonate raises the KH and at the same time supplies potassium to your crops.

Low KH, low and unstable pH

This is the most common case: raise the KH first, and the pH will then stabilise by itself.

Low KH, high pH

This pH is not buffered, so the first acidification will make it drop sharply.

High KH, pH hard to lower

The buffering capacity neutralises the acidifier as you pour it in: first reduce the KH by diluting the water with osmosed water.

A piece of limestone

It barely dissolves above pH 7, which is why it raises the KH so little.

Two causes of a falling KH

Cutting tap water with reverse osmosis water or rainwater lowers the KH in the same proportion as the mixture, and this drop often goes unnoticed until the pH crashes. A technical substrate for a planted tank also binds carbonates, and it lowers the KH for several months, which often leads people to wrongly suspect a faulty test.

Measure GH separately for the minerals needed by living organisms

KH measures carbonates, and therefore buffering capacity, but not calcium or magnesium. If you keep shrimps or snails, or if your plants show a magnesium deficiency, also get the total hardness test: it measures calcium and magnesium, the mineral content that fish, shrimps and plants depend on. The two values have no fixed relationship, so this second figure saves you from correcting the wrong parameter.

☀️ Biogenic decalcification

When carbon dioxide runs short, aquatic plants and especially algae, which absorb it very quickly through photosynthesis, draw carbon from the hydrogencarbonates in the water. This phenomenon, known as biogenic decalcification, lowers the carbonate hardness and pushes the pH up to levels dangerous for fish, most often late on a sunny day in a pond full of algae. A KH that falls without any addition of reverse osmosis water or rain, together with a high pH in the afternoon, therefore points to this mechanism rather than to a faulty reagent. It is one more reason to keep your pond above 5 dKH.

🥽 A corrosive and flammable reagent

The KH reagent contains ethanol, which makes it flammable, and it is classed as corrosive to the skin and eyes: it is the only bottle in the kit, and it carries the Danger signal word. Count your drops over a sink, holding the test tube tilted away from your face, and recap the bottle straight away. Once the colour change is reached, the orange-yellow water goes down the sink: put back in the tank, it would introduce the reagent's acid into the system, when what you are measuring is precisely its capacity to buffer acidity. In aquaponics, where nitrification uses up the carbonates week after week, this bottle runs out faster than in an aquarium, and the KH reagent refill replaces it in the same case.

Technical specifications

Measuring KH

Parameter measured: Carbonate hardness, written as KHDissolved carbonates and hydrogen carbonates, in other words the buffering capacity of the water
Measuring principle: Titration, counting the dropsNeither a fixed range nor a colour chart: you add the reagent until the colour change and count
Value of one drop: 1 °dKHThe number of drops is directly the carbonate hardness in German degrees
Equivalences: 1.78 °fH, 0.36 mmol/LOne drop is also worth 21.8 mg/L of hydrogen carbonate
Minimum value: Never below 5 °dKH5 to 16 °dKH suit most freshwater tanks
Manufacturer's reference: JBL 2411000, PROAQUATEST rangeEAN code 4014162241108

The procedure

Water volume: 5 mlIt sets the value of one drop at 1 °dKH. A different volume changes the rule
The steps: Rinse, fill, add drop by drop, countShake after every drop, otherwise the colour change comes too late
Colour change: From blue to orange-yellowStop at the first drop that tips the colour, not when the yellow becomes clear
Waiting time: NoneThe colour change is immediate, with no waiting time for the colour to develop
Total time: About 2 minutesIn water of ordinary hardness
Water types covered: Freshwater, seawater, pondThe reading rule is the same throughout, since there is no colour chart to swap

Hazard labelling

Signal word: DangerThe single bottle in the kit
Pictograms: Flame and corrosionGHS02 and GHS05
Hazard statements: H226, H314Flammable liquid and vapours, skin burns and eye damage
Precautionary statements: P101, P102, P303+P361+P353, P305+P351+P338, P501Keep the label to show the doctor, children kept away, skin and eyes rinsed, dispose of according to local rules
Precautions: Sink, ventilated room, out of reachBottle recapped straight away, test tube emptied down the sink
Classified component: Ethanol, 10 to 25%It makes the reagent flammable

To allow for in addition

What you need to raise KH: Potassium bicarbonateIt provides alkalinity and leaves potassium for your crops
Reagent refill: Reagent only, ref. 2411100The graduated test tube keeps well

12 further specifications are available in the detailed view.

In the box

1 bottle of KH reagent with child-resistant cap
1 plastic test tube graduated from 5 to 20 ml
1 multilingual instruction leaflet, French included

Manufacturer's values (instructions for use and safety data sheet for the KH reagent). Aquaponic benchmarks from the FAO.

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.

The buffer alone, or the whole kit?

Carbonate hardness is always measured at the same time as pH. The three columns differ in what they cover around this pair.

KH is the buffering capacity that limits the drop in pH Hard tap water has a KH of 10 to 20 °dKH and varies little. Osmosis water starts at 0, so the pH is no longer buffered as soon as nitrification gets going.

	Parameters covered	Drop test kits included	Carbonate hardness KH	pH, nitrite, nitrate and ammonium	Total hardness GH, iron, phosphates, copper, oxygen	Reagent bottles	Glass test tubes	Laboratory comparator block	Thermometer	Reagent refill sold separately	Price
Recommended Carbonate hardness test onlyYour choice	1	1	Yes	No	No	1	0	No	No	Yes	$12.00
6-in-1 Aquarium Water Testing KitYour choice	6	5	Yes	Yes	No	9	3	Yes	No	Yes	$58.00
JBL ProAquaTest Lab water analysis case, 13 tests for aquarium and pondYour choice	12	13	Yes	Yes	Yes	23	12	Yes	Yes	Yes	$114.00

Carbonate hardness test onlyRecommended

What you are measuring

Parameters covered1

Drop test kits included1

Carbonate hardness KHYes

pH, nitrite, nitrate and ammoniumNo

Total hardness GH, iron, phosphates, copper, oxygenNo

Equipment supplied

Reagent bottles1

Glass test tubes0

Laboratory comparator blockNo

ThermometerNo

Budget

Reagent refill sold separatelyYes

6-in-1 Aquarium Water Testing Kit

What you are measuring

Parameters covered6

Drop test kits included5

Carbonate hardness KHYes

pH, nitrite, nitrate and ammoniumYes

Total hardness GH, iron, phosphates, copper, oxygenNo

Equipment supplied

Reagent bottles9

Glass test tubes3

Laboratory comparator blockYes

ThermometerNo

Budget

Reagent refill sold separatelyYes

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

What you are measuring

Parameters covered12

Drop test kits included13

Carbonate hardness KHYes

pH, nitrite, nitrate and ammoniumYes

Total hardness GH, iron, phosphates, copper, oxygenYes

Equipment supplied

Reagent bottles23

Glass test tubes12

Laboratory comparator blockYes

ThermometerYes

Budget

Reagent refill sold separatelyYes

Kit contents as given on the data sheets. A titration needs neither a comparator block nor a glass test tube.

Documents & manuals

Official manufacturer's instructions, in French

Full titration test protocol, reference volume, endpoint reading rule and recommended values by installation type.

PDF · 1,5 Mo

Download

Estimate your
delivery time

Parcel of 0.09 kg, dispatched within 48 h by Colissimo or Chronopost home delivery, as you choose during payment. The reagent is sent in its original box. The exact amount is shown in the basket. Once unsealed, this product can no longer 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

6 questions

6 key questions

My KH is too low, is that serious?

Yes, and it is more urgent than a badly adjusted pH. Below 5 °dKH there are no longer enough carbonates to absorb the acidity continuously produced by your fish and by nitrification: the pH can crash overnight, and a sudden drop kills far faster than a value that is persistently imperfect.

The FAO guide places the aquaponic optimum between 60 and 140 mg/L of calcium carbonate, or roughly 3 to 8 °dKH; keep a margin above 5 °dKH, because nitrification uses up the buffer continuously. The reagent itself stays in the test tube and never goes into the fish's water.

What is the difference between KH and GH?

KH measures the carbonates, that is, the buffer that stabilises your pH. GH measures calcium and magnesium, that is, the mineralisation that fish, shrimp and plants live on.

The two have no fixed relationship: reverse osmosis water remineralised with calcium chloride has a high GH and a KH of zero. Correcting one does not correct the other, which is why the two tests exist separately.

Why does my KH keep dropping on its own?

How can I raise the KH without increasing the GH?

Use a hydrogen carbonate, not a calcium salt.
Potassium bicarbonate raises the KH and supplies potassium to the crops; sodium bicarbonate raises the KH with no other useful benefit.
For 1,000 L, allow roughly 36 g of potassium bicarbonate for each degree to be gained.
Dissolve it separately, pour it in slowly, and do not aim for more than one degree per day.
Retest the next day, never straight away.

A piece of limestone will do almost nothing while your pH stays above 7.

My water turns yellow from the very first drop, is the test unusable?

Is this test used to find out the CO2 level in my planted tank?

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KH water test, carbonate hardness, drop-based

The summary