Cardboard box for the PO4 Sensitive phosphate test, with 50 X TEST printed at the top

Contents of the PO4 phosphate kit: grey comparator block, measuring spoon, sampling syringe, glass test tubes, three reagent bottles and colour chart

Phosphate measurement in progress: test tube placed in the comparator block above the colour chart, with sampling syringe and reagent bottles alongside

The two reagents of the PO4 phosphate test, bottle 1 in powder form and bottle 2 as a dropper

Box of the JBL PO4 phosphate test, Koï version for ponds, 50 measurements, illustrated with koi carp

JBL ProAquaTest phosphate PO4 drop water test

Name: JBL ProAquaTest phosphate PO4 drop water test
Brand: JBL
SKU: J2412700
Price: 15.9 EUR
Availability: InStock

15,90 €

This colorimetric test measures phosphate PO4 in drops, with a comparator block and a reference sample. The Sensitive version reads from 0.05 mg/L for the aquarium and aquaponics, while the Koi version reads up to 10 mg/L for the pond.

Phosphate feeds plants and algae alike, and is read alongside the nitrate.

Ref. J2412700·Water test

Format : Aquarium

Aquarium

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The summary

In the box: 2 or 3 reagent bottles depending on the version, 2 glass test tubes, a syringe, a double measuring scoop, a comparator block and a colour chart.
What it measures: dissolved phosphate, from 0.05 to 1.8 mg/L in the Sensitive version and up to 10 mg/L in the Koi version, with a blank sample that cancels out the water's own tint.
How long it takes: 12 minutes per test, 10 of which are waiting time, and about 50 tests per kit.
The useful thresholds: 0.4 mg/L in a freshwater aquarium, 0.1 mg/L in a marine aquarium and in a garden pond.
How to choose: the Sensitive version for an aquarium or an aquaponic circuit, the Koi version for a heavily stocked pond.
What is sent to you: a small parcel, with the reagents in their original box, delivered to your door by Colissimo or Chronopost.

Drop-based phosphate test, 50 measurements, for aquariums, ponds and aquaponics

JBL's PO4 phosphate drop water test measures dissolved phosphate in 12 minutes, that is 10 minutes of waiting, for about 50 measurements per kit. The Sensitive version reads from 0.05 mg/L for the aquarium and aquaponics, while the Koï version reads up to 10 mg/L for a heavily stocked pond. A control sample compensates for the tint of the water.

Unpolluted natural water contains about 0.01 mg/L of phosphate, whereas a heavily stocked tank can reach 100 times that value without anything giving it away. Phosphate does not cloud the water, has no smell and does not bother any fish at the concentrations found in aquariums, but it is its level, relative to that of nitrogen, that determines how much benefits algae rather than your crops. That is why it is worth measuring, with two versions of the same test: the Sensitive formulation reads from 0.05 mg/L, and the Koï version reads up to 10 mg/L.

🔬 Reading across two test tubes

The measurement is made using two test tubes, one of which receives the reagents while the other holds untreated water and serves as the blank sample in the grey comparator block. This set-up compensates for the natural tint of the water and so gives a usable reading even on water that has been browned by peat filtration or by a treatment, where a single-tube test would return a value that is too high. In the Sensitive version, each tube takes 10 ml and the scale goes up to 1.8 mg/L, which comfortably covers the thresholds you are monitoring: 0.4 mg/L in a freshwater aquarium, 0.1 mg/L in a marine aquarium and 0.1 mg/L in a garden pond.

Dissolve the powder before adding reagent 2

Reagent 1 comes as a powder rather than a liquid, which changes how you handle it. Close the test tube and shake it until fully dissolved before adding the drops of reagent 2, otherwise the colour will stay pale and you will read a value that is too low. Draw the water with the supplied syringe rather than guessing, and let it stand for the full 10 minutes, as the colour only finishes developing after that time.

🐟 Where the phosphate comes from

Most of it comes from the fish's digestion and from uneaten food. Two other sources, however, almost always go unnoticed. Some water companies add phosphates to the supply to limit limescale and corrosion in the pipework, so your top-up water may already contain some straight from the tap. A garden pond also receives phosphates in spring, carried in by pollen and by neighbouring gardens' fertiliser, which explains the algae surges you see without having changed anything in how you run it.

High phosphate, high nitrate

Both come from the same source: the tank receives more food than it uses.

High phosphate, low nitrate

Nitrogen limits plant growth, and phosphate builds up without being used.

Low phosphate, algae present

Algae store it and keep growing even after it has fallen. Today's reading describes the water, not their reserve.

Low phosphate, slow plants

In a heavily planted tank with few fish, it becomes limiting and is dosed like a fertiliser.

Read the phosphate alongside the nitrate

Neither figure on its own settles the matter of an algae outbreak or a culture that is stagnating, since it is their ratio that counts. The nitrate test is done on the same sample and straight afterwards, which takes only 10 minutes more. Get the reagent refill at the same time: only the bottles run out, while your glassware, the syringe, the measuring spoon, the comparator block and the colour chart carry over from one purchase to the next.

🎏 The version for a koi pond

A heavily stocked koi pond receives a large ration every day, and its phosphate level can exceed the ceiling of a formulation calibrated for the aquarium. The Koï version therefore reads a wider range, from less than 0.1 to 10 mg/L, on a colour chart graded in 9 shades. The principle of the control sample is the same: you fill both test tubes with 5 ml of pond water, add one large measuring scoop of powder and then 5 drops to one of them, and read the value after 10 minutes. In a koi pond without plants, aim for a level below 0.1 mg/L.

🌱 Phosphorus in aquaponics

In aquaponics, phosphorus is among the elements that the water supplies most sparingly to plants. The water in a coupled system contains about ten times less than a hydroponic solution, and a good part of what the feed supplies leaves with the solid waste that the mechanical filter removes from the circuit before it can mineralise. Your installation can therefore show a comfortable nitrate level and plants with slowed growth, with older leaves a dark green turning purplish, which are the classic signs of phosphorus deficiency. The guide to deficiencies in aquaponics reviews these signs.

💧 Lowering a high value

Start with the source rather than the fix: cut back the feed, siphon the bottom where solid waste accumulates, then test again after 48 hours before drawing any conclusions. A partial water change will not solve anything if the water you refill with already contains phosphates, which a single test on your tap water will tell you. In a pond, when the source cannot be removed, a phosphate treatment takes over by binding the dissolved fraction. The other ways of tackling green water are gathered in the article on pond algae in summer.

Technical specifications

The essentials

Parameter measured: Phosphate, expressed as PO4A nutrient for plants and algae alike, in freshwater, seawater and ponds
Measurement range: 0.05 to 1.8 or 0.1 to 10 mg/LSensitive to read the lower end of the scale in an aquarium, Koi for a heavily stocked pond
Tests per kit: Approximately 50
Recommended thresholds: 0.4 mg/L in freshwater0.1 mg/L in a marine aquarium and in a garden pond
Method: Powder, drops and block comparatorThe blank test tube cancels out the water's own tint, even on water coloured by peat
Reference: JBL 2412700 or 2407600Sensitive or Koï, PROAQUATEST range

The procedure

Water volume: 2 x 10 or 5 ml10 ml in Sensitive, 5 ml in Koi. One test tube receives the reagents, the other stays with untreated water
Reagents to dose: 1 measuring scoop and 10 or 5 dropsSensitive: small end of the measuring spoon and 10 drops. Koï: large end and 5 drops
The steps: Rinse, fill, add the reagent, dissolve, wait, readIn this order. A poorly dissolved powder gives a pale colour and a value that is too low
Waiting time: 10 minutesTime taken for the colour change to complete. Reading too early gives a low value
Total duration: Approximately 12 minutesHandling included
Reading: In the notch of the blockYou slide the block along the colour chart until the two shades match

Security and parcels

Ranking: DangerReagent 2 is classified. Declared component: hydrogen chloride, i.e. hydrochloric acid
Main risks: Burns, eye injuriesHazard statements H314 and H318. H302 if swallowed, H335 for the respiratory tract
Precautions: Sink, well-ventilated room, out of reachNever smell a bottle or a test tube, and empty the treated test tube down the sink
Parcel weight: Around 0.2 kgPackaging included, for either version

To budget for separately

Reagent refill: Reagents only, ref. 2412800For the Sensitive version: the glassware, syringe, measuring spoon, comparator block and colour chart can all be kept
To read alongside: NitrateIt is the scarcer of the two nutrients that limits growth: phosphate alone does not settle the question

10 further specifications are available in the detailed view.

In the box

2 glass test tubes with screw caps
1 graduated sampling syringe
1 double-ended measure, large and small tip
1 grey comparator block with reading slot
1 colour chart and 1 multilingual instruction leaflet, French included
2 or 3 bottles of reagents, depending on the version chosen

Reference, EAN code, kit composition, number of tests, protocol, thresholds by environment and hazard statements taken from the manufacturer's product sheets and instructions for use, on 19/09/2026 for the Sensitive version and 11/10/2026 for the Koï version. The ten-to-one ratio between the phosphorus in the water of a coupled aquaponic system and that of a hydroponic solution comes from the scientific literature on nutrient cycling in aquaponics.

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.

Phosphate alone, or the rest of the water as well?

Same drop reagents, same comparator block, same manufacturer. What changes is the environment targeted and the number of questions the case answers.

Phosphate alone, or a test case? Two questions about your water and what you want to monitor, and the table highlights itself.

	Recommended Aquarium version (Sensitive)Your choice	6-in-1 Aquarium Water Testing KitYour choice	13-in-1 water testing kit for aquariumsYour choice	Koi pond version, made to orderYour choice
What the kit covers
Parameters covered (parameters)	1 parameters	6 parameters	12 parameters	1 parameters
Drop tests included (tests)	1 tests	5 tests	13 tests	1 tests
Phosphates PO4	Yes	No	Yes	Yes
Nitrates, ammonium, nitrites, pH and carbonate hardness	No	Yes	Yes	No
Iron, copper, oxygen and silicates	No	No	Yes	No
Equipment supplied
Glass test tubes	2	3	12	2
Laboratory comparator block	Yes	Yes	Yes	Yes
Thermometer	No	No	Yes	No
What needs replacing
Reagent refill sold separately	Yes	Yes	Yes	No
Price	15,90 €	50,90 €	99,90 €	15,90 €
Phosphate reading range	0.05 to 1.8 mg/L	Not measured	0.05 to 1.8 mg/L	0.1 to 10 mg/L

Aquarium version (Sensitive)Recommended

What the kit covers

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

Phosphates PO4Yes

Nitrates, ammonium, nitrites, pH and carbonate hardnessNo

Iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes2

Laboratory comparator blockYes

ThermometerNo

What needs replacing

Reagent refill sold separatelyYes

Phosphate reading range0.05 to 1.8 mg/L

6-in-1 Aquarium Water Testing Kit

What the kit covers

Parameters covered (parameters)6 parameters

Drop tests included (tests)5 tests

Phosphates PO4No

Nitrates, ammonium, nitrites, pH and carbonate hardnessYes

Iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes3

Laboratory comparator blockYes

ThermometerNo

What needs replacing

Reagent refill sold separatelyYes

Phosphate reading rangeNot measured

13-in-1 water testing kit for aquariums

What the kit covers

Parameters covered (parameters)12 parameters

Drop tests included (tests)13 tests

Phosphates PO4Yes

Nitrates, ammonium, nitrites, pH and carbonate hardnessYes

Iron, copper, oxygen and silicatesYes

Equipment supplied

Glass test tubes12

Laboratory comparator blockYes

ThermometerYes

What needs replacing

Reagent refill sold separatelyYes

Phosphate reading range0.05 to 1.8 mg/L

Koi pond version, made to order

What the kit covers

Parameters covered (parameters)1 parameters

Drop tests included (tests)1 tests

Phosphates PO4Yes

Nitrates, ammonium, nitrites, pH and carbonate hardnessNo

Iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes2

Laboratory comparator blockYes

ThermometerNo

What needs replacing

Reagent refill sold separatelyNo

Phosphate reading range0.1 to 10 mg/L

Parameters covered, equipment supplied, case contents and reading ranges taken from the manufacturer's data sheets and instructions on 05/09/2026 and 11/10/2026. Sheets compared: 6-in-1 case and 13-in-1 case.

Documents & manuals

Official manufacturer's instructions, in French

Six-step protocol, dosing of the two reagents, recommended thresholds for freshwater, seawater and ponds, and the sources of phosphate in the water.

PDF · 1,6 Mo

Download

Instructions for the Koï version for ponds

Protocol with 2 × 5 ml, dosing of the two reagents and the recommended threshold in a koi pond, multilingual instructions including French.

PDF · 1,6 Mo

Download

Estimate your
delivery time

Small parcel dispatched by Colissimo or Chronopost to your home, as you choose during checkout. The Koï version is ordered for you from the supplier, which adds a few days before dispatch. The exact cost 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

My phosphates are at zero and I still have algae, why?

Because the algae have already built up reserves. They store considerable quantities of phosphate and keep proliferating for some time after the level has dropped. A zero read today describes the water, not the algae.

The second reason is more structural. Algae and plants have adapted to natural levels, around 0.01 mg/L, and make do with very little. Very low-phosphate water therefore does not hold back plants and algae equally: crop growth stops before algal growth does. The useful lever is to work out which of nitrate or phosphate runs short first in your system.

What phosphate level should you maintain to avoid algae?

Where do phosphates in an aquarium come from?

They come from five sources, in this order of importance:

the digestive activity of the fish, the main source;
uneaten food residue;
tap water, when the water company adds phosphates to it;
spring pollen, on a garden pond;
fertiliser from neighbouring gardens, washed down into the pond.

Natural fresh water contains about 0.01 mg/L of phosphate. A heavily stocked tank can contain 100 times more.

Do water companies add phosphates to tap water?

Where should you start to bring phosphates down?

Adjust the ration. Feeding that matches the fish's needs comes before any product.
Siphon off the deposits. What lies stagnant on the bottom keeps releasing phosphate.
Add plants. They consume phosphate without any product to buy again.
Bind the rest with an anti-phosphate treatment, when the source cannot be reduced fast enough.
Re-test the nitrate at the same time. Lowering phosphate further than nitrate, or the other way round, does not give the same result on algae.

Should you aim for zero phosphate?

Sensitive version or Koi version: which one should you choose?

Both versions work on the same principle: powder, drops and a control sample. They differ in three ways:

What changes	Sensitive	Koi
Reading range	0.05 to 1.8 mg/L	0.1 to 10 mg/L
Sample volume	2 x 10 ml	2 x 5 ml
Intended use	Aquarium, aquaponics	Heavily stocked pond

The Sensitive version tells a 0.1 from a 0.4, that is, the two recommended thresholds. The Koi version keeps a usable reading on a heavily loaded pond, where the Sensitive saturates and forces you to dilute the sample.

Do the reagents contain acid?

Yes: the declared component is hydrogen chloride, that is to say hydrochloric acid, a strong mineral acid. It explains the Danger signal word and the H314 and H318 statements, severe skin burns and serious eye damage.

The H335 statement flags possible respiratory irritation, and it changes a habit, that of sniffing a bottle to check something. Work over a sink, in a ventilated room, close the bottles between drops, and empty the treated test tube into the sink rather than into the tank. The H302 statement finally reminds you that the child-resistant cap is no substitute for a cupboard closed and placed up high.

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JBL ProAquaTest phosphate PO4 drop water test

The summary