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

PO4 phosphate water test, drop-based

Name: PO4 phosphate water test, drop-based
Brand: Hardy Dewerse
SKU: J2412700
Price: 27.0 AUD
Availability: InStock

$27.00

$9.00 × 3 with no fees Klarna.

This colorimetric test measures phosphate PO4 by drops, using a comparator block and a reference sample, for aquariums, ponds, and aquaponics. The scale starts at 0.05 mg/L, the range where the useful thresholds fall.

Phosphate feeds both plants and algae, and should be read alongside nitrate.

Ref. J2412700·Water test

Format : Aquarium

Aquarium

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The summary

In the box: 3 reagent bottles, 2 glass test tubes, a syringe, a double measuring spoon, a comparator block and a colour chart.
What it measures: dissolved phosphate, from 0.05 to 1.8 mg/L, using a colour chart, with a blank sample that cancels out the natural tint of the water.
How long it takes: 12 minutes per reading, of which 10 are waiting time, and around 50 readings 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: phosphate is included in the 13-in-1 case, not in the 6-in-1 aquarium one.
What is sent to you: a 0.18 kg parcel, with the reagents in their original box, by Colissimo or Chronopost home delivery.

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

Unpolluted natural water contains around 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 an aquarium, 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 measured, and the Sensitive formulation of this kit reads from 0.05 mg/L.

🔬 Reading across two test tubes

The measurement is taken using two 10 ml test tubes: one 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 turned brown through peat filtration or treatment, where a single-tube test would return a value that is too high. The scale runs 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.

🐟 Where the phosphate comes from

Most of it arrives through the fish's digestion and through uneaten feed, as everyone suspects. 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 phosphates in spring, carried in by pollen and by the fertilisers used in neighbouring gardens, which explains the algae blooms you notice without having changed anything in how you run the pond.

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.

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 10 drops of reagent 2, otherwise the colour will stay pale and you will read a value that is too low. Draw both 10 ml samples with the syringe supplied rather than judging by eye, and let it stand for the full 10 minutes, as the colour only finishes developing after that time.

Read the phosphate alongside the nitrate

Neither figure alone is enough to explain an algae bloom or a crop that is stalling, 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 scoop, the comparator block and the colour chart carry over from one purchase to the next.

🌱 Phosphorus in aquaponics

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

💧 Lowering a high value

Start with the source rather than the remedy: cut back on feeding, siphon the bottom where solid waste accumulates, then measure again after 48 hours before drawing any conclusion. A partial water change will not solve anything if the top-up water already contains phosphates, which a test on your tap water will tell you in a single reading. On a pond, when the source cannot be removed, an anti-phosphate 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.

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 mg/LThe Sensitive formulation reads the bottom of the scale, where the recommended thresholds fall
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, PROAQUATEST range

The procedure

Water volume: 2 times 10 mlOne test tube receives the reagents, the other stays with untreated water
Reagents to dose: 1 measuring scoop and 10 dropsSmall end of the double measuring spoon for reagent 1, the powder
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: DangerBoth bottles of reagent 2 are classified. Declared component: hydrogen chloride, that is 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: 0.18 kgPackaging included

To budget for separately

Reagent refill: Reagents only, ref. 2412800The glassware, the syringe, the measuring scoop, the comparator block and the 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

3 bottles of reagents, including 2 of reagent 2
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

Reference, EAN code, kit contents, number of measurements, protocol, thresholds by medium and hazard statements taken from the product sheet and the manufacturer's instructions for use, on 19/09/2026. Range of 0.05 to 1.8 mg/L taken from the manufacturer's documentation. The tenfold ratio between the phosphorus in coupled aquaponics water and that in a hydroponic solution comes from the scientific literature on nutrient cycling in aquaponics.

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

Same drop reagents, same comparator block, same manufacturer. What changes is the number of questions the kit answers, and phosphate is only included in the larger of the two.

Phosphate alone, or a test case? Two questions about your setup, and the relevant row highlights automatically.

	Parameters covered (parameters)	Drop tests included (tests)	Phosphates PO4	Nitrates, ammonium, nitrites, pH and carbonate hardness	Iron, copper, oxygen and silicates	Glass test tubes	Laboratory comparator block	Thermometer	Reagent refill sold separately	Price
Recommended PO4 phosphate test onlyYour choice	1 parameters	1 tests	Yes	No	No	2	Yes	No	Yes	$27.00
6-in-1 Aquarium Water Testing KitYour choice	6 parameters	5 tests	No	Yes	No	3	Yes	No	Yes	$84.00
13-in-1 water testing kit for aquariumsYour choice	12 parameters	13 tests	Yes	Yes	Yes	12	Yes	Yes	Yes	$197.00

PO4 phosphate test onlyRecommended

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

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

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

Parameters covered, equipment supplied and kit contents taken from the three product pages on 05/09/2026. Product pages compared: 6-in-1 case and 13-in-1 case.

Documents & manuals

Official manufacturer's instructions, in French

Six-step protocol, dosing for both reagents, recommended thresholds in fresh water, salt water and ponds, and sources of phosphate in the water.

PDF · 1,6 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 powder and drops kept separate. The exact amount is shown in your basket.

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 carry on 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 slow plants and algae equally: crop growth stops before algae growth does. The useful lever is to find out which of nitrate or phosphate runs short first in your setup.

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 fishes' digestive activity, the main source;
uneaten food residue;
tap water, when the water company adds phosphates to it;
spring pollen, in a garden pond;
fertilisers 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 as much.

Do water companies add phosphates to tap water?

Where should you start to bring phosphates down?

Adjust the feeding. Feeding that matches the fish's needs comes before any product.
Siphon off the deposits. What sits on the bottom keeps releasing phosphate.
Add plants. They take up phosphate without any product to buy again.
Lock up the rest with a phosphate remover, when the source cannot be reduced quickly enough.
Re-test nitrate at the same time. Lowering phosphate more than nitrate, or the other way round, does not have the same effect on algae.

Should you aim for zero phosphate?

What does Sensitive mean on the box?

The word means the formulation is calibrated for low concentrations. It comes down to three points:

What changes	Value	What it gives you
First readable step	0.05 mg/L	Tells a 0.1 from a 0.4, that is, the two recommended thresholds
Top of the range	1.8 mg/L	A heavily loaded pond saturates the scale: you need to dilute the sample
Sample volume	10 ml	Twice as much as on an ammonium test, which helps the precision of the reading

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 and H318 statements, 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 well-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 kept closed and out of reach.

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PO4 phosphate water test, drop-based

The summary