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

Phosphate PO4 water test in drops

Name: Phosphate PO4 water test in drops
Brand: JBL
SKU: J2412700
Price: 18.05 USD
Availability: InStock

$18.05

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

Version : Aquarium (sensitive)

Aquarium (Sensitive)

Koi pond, to order

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Delivery within 2 to 15 business days

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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 Koï version, with a control sample that cancels out the water's own tint.
How long it takes: 12 minutes per measurement, including 10 minutes of waiting, and about 50 measurements 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 system, the Koï version for a heavily stocked pond.
Handling: bottle 2 contains hydrochloric acid; each dosing is done over a sink, and no drop ever goes into the fish's water.

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

This drop test measures dissolved phosphate, from 0.05 mg/L in the Sensitive version for the aquarium and aquaponics, up to 10 mg/L in the Koï version for the pond, with about 50 measurements. Its acid reagent stays in the test tube, never in the fish water. The PO4 refill reuses the same glassware, version by version.

Unpolluted natural water contains around 0.01 mg/L of phosphate, whereas a heavily stocked tank can reach 100 times that figure with nothing to give it away. Phosphate does not cloud the water, has no smell and does not bother any fish at the concentrations found in an aquarium, but it is its level, relative to that of nitrogen, that determines how much goes to the algae rather than to 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 Koi version reads up to 10 mg/L.

🔬 Reading with two test tubes

The measurement is made using two test tubes, one of which receives the reagents while the other holds the untreated water and serves as a blank sample in the grey comparator block. This set-up compensates for the water's own tint and so gives a usable reading even on water that has been stained brown 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 receives 10 ml and the scale goes up to 1.8 mg/L, which comfortably covers the thresholds you monitor: 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 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 the powder has dissolved completely before adding the drops of reagent 2, otherwise the colour will stay pale and you will read a value that is too low. Draw up the water with the syringe supplied rather than guessing the amount, and leave it to stand for the full 10 minutes, as the colour only finishes developing after that time.

🐟 Where 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 pipes, so your top-up water may already contain some straight from the tap. A garden pond also receives phosphate in spring, carried in by pollen and by the neighbours' fertilisers, which explains the algae blooms you notice 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 consumes.

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 dropped. Today's figure describes the water, not their reserve.

Low phosphate, slow-growing plants

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

Reading phosphate alongside nitrate

Neither figure on its own tells you whether an algae bloom or a stalled crop is to blame, since it is their ratio that matters. The nitrate test is done on the same sample straight afterwards, which takes only 10 more minutes. Take 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 content can exceed the ceiling of a formulation calibrated for the aquarium. The Koi version therefore reads a wider range, from under 0.1 to 10 mg/L, on a colour chart graded in 9 shades. The principle of the blank sample remains the same: fill both test tubes with 5 ml of pond water, add one large 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 one of the elements that the water supplies most sparingly to plants. The water of a coupled system contains roughly ten times less than a hydroponic solution, and a good share of what the feed provides leaves with the solid waste that the mechanical filter removes from the circuit before it has mineralised. Your system can therefore show a comfortable nitrate level alongside slow-growing plants whose older leaves turn a dark green shading to purple, the classic signs of phosphorus deficiency. The guide to deficiencies in aquaponics runs through these signs.

💧 Bringing a high value down

Start with the source rather than the fix: cut back on feeding, siphon the bottom where solid waste builds up, then re-test after 48 hours before drawing any conclusion. A partial water change will not solve anything if the top-up water already contains phosphate, which a single test on your tap water will tell you. On a pond, when the source cannot be eliminated, a phosphate-removal treatment takes over by binding the dissolved fraction. The other ways of tackling green water are brought together in the article on pond algae in summer.

⚗️ The acid in bottle 2

The second reagent contains hydrochloric acid, declared as hydrogen chloride: it burns the skin and eyes, irritates the respiratory tract and is harmful if swallowed. The first, an ascorbic acid powder, is not classified. Work standing over a sink, with the bottle held away from your face, and never put a dosed test tube back into the tank or pond. In aquaponics, where you are on the contrary trying to keep phosphorus for the crops, a sample poured back would bring acid and reagents to the fish and the roots alike. The two test tubes, the measuring spoon and the block are kept, and the PO4 reagent refill replaces only the bottles.

Technical specifications

Measuring phosphate

Parameter measured: Phosphate, expressed as PO4A nutrient for plants and algae alike, in freshwater, marine tanks and ponds
Measuring range: 0.05 to 1.8 or 0.1 to 10 mg/LSensitive for reading the low end of the scale in an aquarium, Koi for a heavily stocked pond
Tests per kit: Around 50
Recommended levels: 0.4 mg/L in freshwater0.1 mg/L in a marine aquarium and in a garden pond
Method: Powder, drops and comparator blockThe control test tube cancels out the water's own colour, even in water stained by peat
Reference: JBL 2412700 or 2407600Sensitive or Koï, PROAQUATEST range

The procedure

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

Hazard labelling

Signal word: DangerReagent 2 in both versions; the powdered reagent 1 is not classified
Pictograms: Corrosion and exclamation markGHS05 and GHS07
Hazard statements: H302, H314, H335Harmful if swallowed, burns to the skin and eyes, irritation of the respiratory tract; declared component of reagent 2: hydrochloric acid
Precautionary statements: P101, P102, P303+P361+P353, P305+P351+P338, P501Keep the label for the doctor, out of reach of children, skin and eyes rinsed, dispose of according to local rules
Precautions: Sink, ventilated room, out of reachNever sniff a bottle, and empty the dosed test tube down the sink rather than into the tank water
Classified component: Hydrochloric acidDeclared as hydrogen chloride, in reagent 2 of both versions

To allow for in addition

Reagent refills: Reagents only, ref. 2412800For the Sensitive version: the glassware, syringe, measuring scoop, comparator block and colour chart can all be kept
Read with: NitrateIt is the scarcer of the two nutrients that limits growth: phosphate alone is not conclusive

12 further specifications are available in the detailed view.

In the box

2 glass test tubes with screw caps
1 graduated sampling syringe
1 double measuring spoon, large and small end
1 grey comparator block with reading notch
1 colour chart and 1 multilingual instruction leaflet, French included
2 or 3 reagent bottles, depending on the version chosen

Manufacturer's values (instructions for both versions, colour charts and safety data sheets).

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?

The same drop reagents, the same comparator block, the same manufacturer. What changes is the environment each is aimed at and the number of questions the kit 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	JBL ProAquaTest Lab water analysis case, 13 tests for aquarium and pondYour choice	Koi pond version, made to orderYour choice
What the kit covers
Parameters covered	1	6	12	1
Drop test kits included	1	5	13	1
Phosphates PO4	Yes	No	Yes	Yes
Nitrate, ammonium, nitrite, 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 can be bought separately
Reagent refill sold separately	Yes	Yes	Yes	No
Price	$18.05	$57.77	$113.38	$18.05
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 covered1

Drop test kits included1

Phosphates PO4Yes

Nitrate, ammonium, nitrite, pH and carbonate hardnessNo

Iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes2

Laboratory comparator blockYes

ThermometerNo

What can be bought separately

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 covered6

Drop test kits included5

Phosphates PO4No

Nitrate, ammonium, nitrite, pH and carbonate hardnessYes

Iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes3

Laboratory comparator blockYes

ThermometerNo

What can be bought separately

Reagent refill sold separatelyYes

Phosphate reading rangeNot measured

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

What the kit covers

Parameters covered12

Drop test kits included13

Phosphates PO4Yes

Nitrate, ammonium, nitrite, pH and carbonate hardnessYes

Iron, copper, oxygen and silicatesYes

Equipment supplied

Glass test tubes12

Laboratory comparator blockYes

ThermometerYes

What can be bought separately

Reagent refill sold separatelyYes

Phosphate reading range0.05 to 1.8 mg/L

Koi pond version, made to order

What the kit covers

Parameters covered1

Drop test kits included1

Phosphates PO4Yes

Nitrate, ammonium, nitrite, pH and carbonate hardnessNo

Iron, copper, oxygen and silicatesNo

Equipment supplied

Glass test tubes2

Laboratory comparator blockYes

ThermometerNo

What can be bought separately

Reagent refill sold separatelyNo

Phosphate reading range0.1 to 10 mg/L

Ranges, equipment and contents taken from the data sheets and instructions of the products compared.

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 of about 0.2 kg, shipped 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 amount is shown in your 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

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 amounts of phosphate and carry on proliferating for some time after the level drops. A zero reading 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 algae growth does. The useful lever is knowing which of nitrate or phosphate runs short first in your system.

What phosphate level should be maintained 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;
residues of uneaten food;
tap water, when the water company adds phosphates to it;
spring pollen, on a garden pond;
fertilisers from neighbouring gardens, washed down into the pond.

Natural freshwater 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 I start to bring phosphates down?

Adjust the ration. Feeding in line with your fish's needs comes before any product.
Siphon off deposits. Whatever settles on the bottom keeps releasing phosphate.
Add plants. They take up phosphate without any product to buy.
Bind the remainder with an anti-phosphate treatment, when the source cannot be reduced quickly enough.
Re-test the nitrate at the same time. Lowering phosphate more than nitrate, or the other way round, does not give the same result on algae.

Should you aim for zero phosphate?

Sensitive version or Koï version, which should I choose?

Both versions are based 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 still gives a usable reading in a heavily loaded pond, where the Sensitive one saturates and forces you to dilute the sample.

Do the reagents contain acid?

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

The H335 statement flags possible respiratory irritation, and it changes one habit, that of sniffing a bottle to check something. Work over a sink, in a ventilated room, close the bottles between drops, and tip the treated test tube down the sink: a test reagent never goes into the fish water. Finally, the H302 statement is a reminder that the child-resistant cap is no substitute for a cupboard kept closed and out of reach.

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Phosphate PO4 water test in drops

The summary