We use analytics cookies to understand how visitors use this page, so we can improve it. No advertising or tracking cookies are used. You can accept or decline below. Manage preferences.
Cookie preferences
Choose which cookies you allow. Necessary cookies cannot be disabled as they are required for the page to function.
Necessary
Essential for the page to work correctly. Always active.
Always on
Analytics
Helps us understand how visitors use this page (Google Analytics via GTM). No personal data is shared with advertisers.
Instructions for Use
How to use Phoslock
Phosphate (PO₄) is the fertiliser that feeds algae growth.
Algaecides may kill visible algae, but if the nutrients remain, algae and blanket weed can return.
Phoslock locks up phosphate, helping remove the root cause of nuisance algae growth.
Phoslock Calculator
Calculate your recommended dosage
PO₄ → Locked
Measure your water's phosphate level using a good quality PO₄ test kit, then use the calculator below to determine the recommended amount of Phoslock required for your pond or aquarium.
Calculator mode: Pond. Use the buttons above to switch between Pond, Freshwater Aquarium and Saltwater / Reef Aquarium.
Corals and invertebrates can be sensitive to rapid PO₄ changes — reduce phosphate slowly and test regularly.
Enter your pond or aquarium volume in litres.
Enter length, width and average depth to calculate volume.
Enter your phosphate test result in mg/L or ppm PO₄.
PO₄ in system0 g
Recommended Phoslock0 g
You will need—
Enter your water volume and measured PO₄ level to calculate the recommended Phoslock dose.
Reef aquarium note: reduce phosphate slowly. We recommend staged dosing and regular PO₄ testing rather than a single rapid reduction.
Pond instructions
1
Mix
Mix Phoslock with pond water in a bucket or watering can with the rose removed. The mixture will form a cloudy slurry — this is normal.
2
Apply
Pour evenly across the pond surface. As the mixture descends through the water column, Phoslock binds with available phosphate. Cloudiness should clear shortly after treatment.
3
Settle & Lock
Phoslock settles to form a sediment cap, absorbing phosphate and permanently locking it so it cannot be released back into the water.
TIP: Remove as much algae and blanket weed as possible before using Phoslock. Phoslock does not kill or remove algae, but instead slows or stops its growth by removing available phosphate (PO₄).
PO₄ → LockedTargets algae's food sourceWorks in water & sediment
Surplus product remains useful. Any Phoslock not immediately used to bind phosphate can remain available to bind new phosphate as it becomes available over time.
Frequently asked questions
When should I use Phoslock?
Phoslock can be used year-round. It works by binding soluble phosphate rather than organic phosphorus already held inside algae. Applications in late autumn or winter can reduce phosphate before spring algae growth. Applications in spring or summer will not kill algae already present, but will help reduce further growth by removing available phosphate.
How much Phoslock should I apply?
As a general pond dose, apply up to 250 mg of Phoslock per litre of pond water. This can be applied as a single dose, or split into two applications: 150 mg/L initially, followed by 100 mg/L after 4–6 weeks.
Example: a 5,000 litre pond would require up to 1.25 kg of Phoslock.
Will this dose always be enough?
Ponds with very high phosphate levels or continuing nutrient inputs, such as run-off, may require additional applications over time.
When should I reapply?
Consider reapplying when algae or blanket weed levels begin to increase again, as this may indicate that available Phoslock has become saturated or new phosphate is entering the pond.
Should I use Phoslock when topping up my pond?
As water evaporates from your pond, chemicals like phosphate can become concentrated. If you top up with tap water, you may also be adding more phosphate, as many water companies add phosphate to help protect customers from lead in older pipe systems.
Tap water phosphate levels are commonly between 1–2 mg/L PO₄, which is enough to help promote algae growth in ponds.
Freshwater aquarium instructions
1
Mix
Mix 50 g of Phoslock with 400 ml of aquarium water in a suitable container. Stir until a smooth, lump-free mixture is formed. Do not use a magnetic stirrer.
2
Apply
Pour evenly across the aquarium water surface. For initial dosing, turn off circulation pumps for 60–90 minutes while keeping filters active.
3
Settle & Lock
Phoslock binds available phosphate and settles to help lock phosphate in the substrate, reducing its availability to nuisance algae.
The prepared Phoslock mixture can be stored in a cool, dry place and used for up to 30 days after preparation.
Temporary cloudiness during initial dosing is normal and may last 24–48 hours depending on filtration. Clean the glass if needed. If cloudiness persists, a normal aquarium flocculant may be used.
Frequently asked questions
Will Phoslock affect my fish?
Phoslock is designed to bind phosphate rather than act as an algaecide. In normal use it is considered safe for freshwater aquarium fish when dosed gradually and used as directed.
Why has my aquarium gone cloudy?
Temporary cloudiness is normal after dosing as the Phoslock settles and binds phosphate from the water column. This normally clears within 24–48 hours depending on filtration.
Can I use Phoslock with plants?
Yes. However, phosphate is also a nutrient used by aquatic plants, so phosphate should be reduced gradually and monitored rather than stripped too quickly.
Can I overdose Phoslock?
Unused Phoslock remains available to bind phosphate as it becomes available. We still recommend staged dosing and regular PO₄ testing rather than making large adjustments all at once.
How low should I reduce phosphate?
For many freshwater aquariums a target around 0.05 ppm PO₄ is commonly used to help reduce nuisance algae while still supporting a stable aquarium environment.
Example dosing schedule
Measure and record the aquarium phosphate level before dosing.
As a guide, for a 500 L freshwater aquarium at 1 ppm PO₄, add 25 ml of the prepared mixture.
After 24 hours, test and record the phosphate level again.
Repeat dosing as required until the target phosphate level is reached.
Adjust the dose depending on how quickly you want phosphate to fall.
We recommend reducing phosphate gradually over around 4 weeks to reach 0.05 ppm PO₄.
Once 0.05 ppm PO₄ is reached, stop dosing for 1 week, then retest and dose again only if required.
Saltwater / reef aquarium instructions
1
Mix
Mix 100 g of Phoslock with 400 ml of aquarium water in a suitable container. Stir until a smooth, lump-free mixture is formed. Do not use a magnetic stirrer.
2
Apply
Pour evenly across the water surface, sump, or refugium. For initial dosing, turn off circulation pumps for 60–90 minutes while keeping filters active.
3
Settle & Lock
Phoslock binds available phosphate and settles to help lock phosphate in the substrate, reducing its availability to nuisance algae.
Important: Corals and other invertebrates can be sensitive to rapid phosphate changes. Reduce available phosphate slowly and monitor PO₄ throughout treatment.
Temporary cloudiness during initial dosing is normal and may last 24–48 hours depending on filtration. Clean the glass if needed. If cloudiness persists, a normal aquarium flocculant may be used.
Frequently asked questions
Will Phoslock affect corals?
Corals and other invertebrates can react to rapid phosphate reduction. We recommend reducing PO₄ slowly and testing regularly throughout treatment.
Why is my reef aquarium cloudy after dosing?
Temporary cloudiness is normal while Phoslock disperses and settles through the aquarium. This usually clears within 24–48 hours depending on filtration and circulation.
Can I add Phoslock into the sump?
Yes. Phoslock can be added to a sump or refugium, ideally in an area with good water movement and circulation.
How quickly should I lower phosphate in a reef aquarium?
Slowly. Rapid PO₄ reduction can stress corals and destabilise reef systems. Gradual staged dosing with regular testing is strongly recommended.
What phosphate level should I aim for?
Many reef keepers target around 0.05 ppm PO₄; however, ideal phosphate levels can vary depending on coral type, nutrient strategy and aquarium maturity.
Example dosing schedule
Measure and record the aquarium phosphate level before dosing.
As a guide, for a 500 L saltwater aquarium at 1 ppm PO₄, add 50 ml of the prepared mixture.
After 24 hours, test and record the phosphate level again.
Repeat dosing as required until the target phosphate level is reached.
Adjust the dose depending on how quickly you want phosphate to fall.
We recommend reducing phosphate gradually over around 4 weeks to reach 0.05 ppm PO₄.
Once 0.05 ppm PO₄ is reached, stop dosing for 1 week, then retest and dose again only if required.
The mixture can be dosed manually or with a peristaltic pump, preferably at the beginning of the night period in an area of high water circulation.
Treatments using the same prepared mixture for more than 30 days are not recommended. If phosphate remains high after 30 days, retest and restart the calculator using the new phosphate level.