The best blanketweed treatments, compared.
Top pick: Cloverleaf Pre-Vent 800 g Best value: Cloverleaf Blanket Answer 4 kg – About £1.30 per 1,000 litres of a stated 50,000 litres, at about £64.99, with the pump left on. Updated October 2026.
Blanketweed treatment on amazon.co.uk is a powder, a liquid, or a treatment-plus-prevention pack for hair-like filamentous algae, also called silkweed. Buyers are goldfish and koi keepers, wildlife-pond owners, and people whose pump clogs in summer. The Royal Horticultural Society separates blanketweed from pea-soup green water, and tells you to twirl mats out, shade the surface, and use barley straw only to discourage new growth. A shop tub comes after that check, not instead of it. Quality is a stated volume that matches your pond, a dose you can measure, and reviews that are actually about blanketweed. Litres, dose style and who should skip a tub are set out in the [blanketweed treatment buying guide](/hub/garden-ponds/blanketweed-treatment/sub/blanketweed-treatment-buying-guide). It is not a treatment for pea-soup green water, duckweed, or a submerged plant you meant to keep. Hair-like mats that you can twirl onto a stick are blanketweed. Early signs are covered in [warning signs of blanketweed](/hub/garden-ponds/blog/blanketweed-warning-signs).
Blanketweed treatment on amazon.co.uk is a powder, a liquid, or a treatment-plus-prevention pack for hair-like filamentous algae, also called silkweed.
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Personal buying guide
Question 1 of 2
What is growing in the pond?
Cloverleaf Pre-Vent 800 g at about £21.79 is sold as a follow-up to Blanket Answer, not as the tub that clears a thick mat. It is 4.9 from 27 amazon.co.uk reviews, the highest star line in this set and a small sample beside 4,486 ratings on the older Blanket Answer listing. The bullets say every 10 to 14 days, best above 10°C, and safe for pond fish, plants, and biological filtration. The page does not state a treat volume. One reviewer mixes the powder with pond water before adding it. A 4.9 from 27 does not outrank a clear-out product that has thousands of pond reviews. Use it after the mat has already been treated, and skip it if the weed is still a solid blanket.
Pros
Cons
Best for: You have already used Blanket Answer and you will repeat Pre-Vent every 10 to 14 days above 10°C.
Less suitable: You are buying your first tub for a solid mat, or you need a litre figure on the page.
Specifications
Common questions
Is Pre-Vent the product that clears a thick mat?
The listing sells it as a follow-up to Blanket Answer, every 10 to 14 days, to stop blanketweed returning.
What temperature does the listing mention?
Best results above 10°C.
How many reviews are on the saved page?
27, at 4.9. That is a much smaller sample than the 800 g Blanket Answer listing.
Cloverleaf Blanket Answer 800 g, model BA800GM, is about £19.49 and states up to 10,000 litres of powder. It is 4.7 from 258 amazon.co.uk reviews, a higher star line than the older 800 g listing and a much smaller sample than that listing's 4,486. The bullets name a powder, a plastic measuring spoon, and no need to turn off the pump or filters, and they list koi, goldfish, and freshwater pond fish. Reviewers mix it with pond water in a watering can, and some still pause the pump for about an hour. Some use it monthly. One reviewer says dying algae uses oxygen, so they add air and avoid very hot weather. The cleaner form line is the reason to notice this ASIN, while the older listing remains the larger review file.
Pros
Cons
Best for: You want the 800 g powder with the spoon named on the page, and your pond is near 10,000 litres.
Less suitable: You are choosing only by the highest star line, or the pond is far larger than 10,000 litres.
Specifications
Common questions
How much water does BA800GM claim to treat?
The bullets say up to 10,000 litres. The pack is 800 g of powder.
Does the listing say to turn the filter off?
The bullets say there is no need to turn off the pump or filters. Some reviewers still pause the pump for about an hour.
What do reviewers say about oxygen?
One reviewer says dying algae uses oxygen, so add air and avoid very hot weather.
Cloverleaf Blanket Answer 4 kg at about £64.99 is a powder stated to treat 50,000 litres, and the bullets and the liquid-volume field agree. It is 4.6 from 111 amazon.co.uk reviews, a much smaller file than the 800 g listing's 4,486. The bullets say you do not need to turn off the pump or filters, and that it is safe for pond fish, plants, pets, and animals. A plastic measuring spoon is listed, though some reviewers say it was missing or that the pack arrived as two 2 kg buckets. Reviewers still say to take out as much blanketweed as you can first and to clean the filter afterwards. One reviewer doses heavily in spring above 10°C, and the bullets do not print that minimum. At about £1.30 per 1,000 litres of stated capacity it is the large-pond buy in this set, provided the pond is actually near that 50,000 litre claim.
Pros
Cons
Best for: You have a large fish pond near the stated 50,000 litres and you will net weed and rinse the filter.
Less suitable: You have a patio pond under 10,000 litres, where the 800 g tub matches the size without leftover powder.
Specifications
Common questions
How much water does the 4 kg tub claim to treat?
The bullets and the liquid-volume field both say 50,000 litres. Item weight is 4 kg.
Does the listing say to switch the pump off?
No. The bullets say there is no need to turn off the pump or filters.
What do reviewers say to do afterwards?
Clean the filter. Several also say remove as much blanketweed as you can before dosing.
Cloverleaf Blanket Answer 800 g at about £19.47 is listed for ponds up to 10,000 litres, and that figure agrees on the bullets and the liquid-volume field. Across 4.5 from 4,486 amazon.co.uk reviews, the usual account is milky water for a day or several days, then weed breaking down. The maker's dose is 10 g per 125 litres, with the cloud expected for about five days and the UV left on. The item-form field says liquid and powder together, while the title and the 800 g weight are a powder tub, and a measuring spoon is not listed as included. Several reviewers say the dying blanketweed turns to sludge and blocks the pump, so the filter needs cleaning more often. People describe one dose, a spring dose, or a monthly top-up, which is a wider habit than a single promise. The star line is the largest pond sample in this set, and it is not a laboratory test.
Pros
Cons
Best for: You have a fish pond around the stated 10,000 litres and you can wait out milky water while you clean the filter.
Less suitable: You want a clear pond the same afternoon, or you will not open the pump when the sludge arrives.
Specifications
Common questions
How much water does this 800 g tub claim to treat?
The listing says ponds up to 10,000 litres. Item weight is 800 g.
What do reviewers say happens to the water?
Many describe milky water for a day or several days, then clearer water and weed breaking down.
Do reviewers mention the pump?
Several say dying blanketweed turns to sludge and blocks the pump or filter, so they cleaned it more often.
Cloverleaf Blanket Answer 500 g at about £16.75 states up to 6,250 litres on the bullets. It is 4.5 from 83 amazon.co.uk reviews. The form is described as powdery and granular, while a liquid-volume field says 500 millilitres, which clashes with the 500 g weight. The bullets say it is safe for pond fish, plants, pets, and animals, and that the pump and filters can stay on. A plastic measuring spoon is listed, and one reviewer did not receive one. Reviewers mention a few doses rather than a single sprinkle that finishes the year. At about £2.68 per 1,000 litres of stated capacity it costs more per litre than the 800 g tub, so the saving is the smaller pack, not a cheaper treatment.
Pros
Cons
Best for: You have a pond near the stated 6,250 litres and you do not want an 800 g tub left open.
Less suitable: You will treat the 500 millilitre field as the dose, or you expected the large review file of the older 800 g listing.
Specifications
Common questions
How much water does the 500 g tub claim to treat?
The bullets say up to 6,250 litres. A liquid-volume field says 500 millilitres, which clashes with the 500 g powder. Do not dose from that field.
Does the listing say it suits fish?
The bullets say it is safe for pond fish and plants, and for pets and animals.
Is a spoon included?
The listing says a plastic measuring spoon. One reviewer did not receive one.
The Cloverleaf Blanket Answer and Pre-Vent bundle at about £40.01 is titled as treatment and prevention, with an item weight of 800 g. It is 4.5 from 12 amazon.co.uk reviews. The saved page has no feature bullets, no litre figure, and no split between the two products. Reviewers describe using one product and then the other through the year. In one heavy case the filter was cleaned once or twice a day while the weed died. Buying both jobs in one listing only helps if you already know which tub is the clear-out and which is the follow-up. At about £40.01 it costs about as much as Blanket Answer at about £19.47 plus Pre-Vent at about £21.79, without telling you the grams of each.
Pros
Cons
Best for: You already plan to use Blanket Answer and then Pre-Vent, and you accept an unstated split.
Less suitable: You need a stated treat volume before you pay, or you only have a thick mat and no plan for a follow-up.
Specifications
Common questions
What is in the 800 g bundle?
The title says Blanket Answer and Pre-Vent. The page does not say how many grams of each.
How much water does it treat?
Not stated. The saved listing does not give a litre figure.
What do the few reviewers describe?
Using one product then the other. One heavy case needed the filter cleaned once or twice a day while the weed died.
Evolution Aqua Stop Blanketweed 4 kg at about £39.99 is listed at 4.2 from 1,036 amazon.co.uk reviews. The title is a 4 kg tub and reviewers describe powder or granules, while the item-form field says liquid and the liquid-volume field says 1 millilitre, so neither of those cells is a dose. The listing does not state a treat volume. The maker's page uses 30 g per 900 litres once a week for about three weeks, then monthly, and it prints HSE No. 9647 as Evolution Aqua's own statement, which this comparison does not certify. Reviewers stir it into a watering can because it does not dissolve easily, and many describe weekly treatments rather than one pour. The bullets say it is safe for fish and pond plants and that it prevents regrowth. One reviewer applied it at 9°C, and the maker says it is weaker below 8°C. A weekly course means the about £39.99 is not the cost of a single clear-out.
Pros
Cons
Best for: You can run a weekly course and you will follow 30 g per 900 litres on the label, not the 1 millilitre cell.
Less suitable: You want one dose this weekend, or you need a litre figure printed on the amazon.co.uk page before you buy.
Specifications
Common questions
How many litres does this listing say the 4 kg tub treats?
It does not. The liquid-volume field says 1 millilitre, which clashes with the 4 kg weight. That cell is not a dose. The maker's page uses 30 g per 900 litres.
What form is it?
The item-form field says liquid. The title says 4 kg and reviewers describe powder or granules that are hard to dissolve.
Do reviewers dose once?
Many describe weekly treatments for several weeks, not a single dose.
envii Algae Klear Xtra at about £20.95 is a 500 ml liquid, model AKX01, listed at 4.1 from 4,188 amazon.co.uk reviews. The bullets say it treats 10,000 litres of submerged blanketweed and string algae, and they aim for a result within three weeks of the first treatment. A separate liquid-volume field on the same page says 20,000 litres, so that cell is not the figure to dose from. The maker's page states 50 ml per 1,000 litres and lists copper sulfate, and it sends floating mats to a different envii product. The bullets say it is safe for all fish and plants, while reviews are mixed, including one report of newt deaths, which is a single account rather than a counted rate. The formula line mentions a UV light filter, and the bullets say it can be used all year, with no repeat interval printed. It is a poor first bottle if the mat is sitting on the surface or if the pond holds newts.
Pros
Cons
Best for: You have submerged string algae in a fish pond near the stated 10,000 litres, and you can follow 50 ml per 1,000 litres.
Less suitable: You have a floating mat, newts in the pond, or you would treat the 20,000 litre field as the dose.
Specifications
Common questions
How much water does the listing say a 500 ml bottle treats?
The bullets say 10,000 litres. A separate liquid-volume field on the same page says 20,000 litres. Dose from the bullets and the maker's 50 ml per 1,000 litres, not from the clashing field.
Does the listing say it is safe for fish?
The bullets say it is completely safe for all fish and plants. Reviews are mixed, and one reviewer reports newt deaths. That is one account, not a counted rate.
Does the listing limit the season?
The bullets say it can be used all year round and aim for a result within 3 weeks of the first treatment.
NT Labs Blanketweed Balance 800 g at about £13.39 is the lowest shelf price in this set, and the title says it treats 10,000 litres by re-mineralising the water. It is 4.1 from 15 amazon.co.uk ratings. Several of those saved reviews discuss brine shrimp, so the star line cannot be read as 15 pond treatments. The bullets say to aerate, test the water, and mix the dose in a bucket or watering can of pond water, and they describe a cloud that clears after a few hours. A liquid-volume field says 2 kilograms, which clashes with the 800 g weight. One on-topic reviewer needed four or five applications. The maker says not to dose at daybreak, in hot or thundery weather, or within three hours of feeding. The rank sits in Fish Food, which matches the stray shrimp reviews better than a pond-treatment sample.
Pros
Cons
Best for: You want a mineral powder for a pond near 10,000 litres and you will ignore the shrimp reviews and the 2 kilogram cell.
Less suitable: You are choosing the cheapest tub because of the star line, or you cannot aerate before you dose.
Specifications
Common questions
How much water does the title say 800 g treats?
10,000 litres. A liquid-volume field says 2 kilograms, which clashes with the 800 g weight. Do not use that field as the dose.
What do the bullets say to do first?
Aerate the pond, test the water, and mix the dose in pond water. A cloud that clears in a few hours is described as normal.
Can the 15 ratings be read at face value?
Not on this saved page. Several reviews discuss brine shrimp, not blanketweed.
Evolution Aqua's 2 kg blanketweed powder at about £25.49 states 25,000 litres on the title and on the liquid-volume field. It is 4.0 from 445 amazon.co.uk reviews. The bullets cover blanketweed, pond weed, and algae, and one bullet also says duckweed treatment, so the job is wider than a single hair-like mat. They call it pond-safe for fish, plants, and wildlife, and safe with biological filtration. A reviewer quotes one scoop per 400 litres, and the bullets do not print that rate or a repeat interval. Accounts include a second dose after 14 days or monthly use, and reviewers still report blocked filters. A run of reviews says it did nothing, so the 25,000 litre headline is not a result. The maker's natural-powder page uses one scoop per 400 litres every 14 days, with cloudiness for up to five days, which is a different schedule from Stop Blanketweed's weekly 30 g.
Pros
Cons
Best for: You want the stated 25,000 litre powder and you will follow the scoop on the label every 14 days.
Less suitable: You need one weekend clear-out, or duckweed is the only plant you are trying to shift.
Specifications
Common questions
How much water does this 2 kg pack claim to treat?
The title and the liquid-volume field say 25,000 litres.
Does the listing mention the filter?
The bullets say it is safe with biological filtration. Reviewers still report blocked filters.
How often do reviewers repeat it?
Accounts include every 14 days and monthly use. The bullets do not print an interval. The maker's page uses one scoop per 400 litres every 14 days.
The Cloverleaf Blanket Answer and Pre-Vent bundle at about £79.99 is a 2 kg listing for treatment and prevention. The saved page shows 3.0 from 1 rating and no review text. It does not say how the 2 kg is split, and it prints no litre figure, no fish line, and no dose. Country of origin is the United Kingdom, and the rank is 850 in Water Garden and Pond Treatment, far below the single 800 g Blanket Answer tub. At about £79.99 you pay more than the 4 kg Blanket Answer at about £64.99, which at least states 50,000 litres and has 111 reviews. There is no review sentence here to tell you whether the water went milky or the filter blocked. It is a bundle name without a specification you can match to a pond.
Pros
Cons
Best for: You only need to see that this 2 kg bundle does not state a split or a volume.
Less suitable: You want a reviewed tub with a litre figure, which the single-size Cloverleaf listings provide.
Specifications
Common questions
What is this listing?
The title says a Cloverleaf Blanket Answer and Pre-Vent bundle, 2 kg, for treatment and prevention.
How is the 2 kg split?
Not stated. The saved page does not say how the grams are divided.
What is the rating?
The saved page shows 3.0 from 1 rating and no review text.
Cloverleaf Blanket Answer 25 kg at about £284.28 states up to 312,500 litres on the bullets. The saved amazon.co.uk page has no customer-review line, so there is no star rating and no review text to judge the tub by. The bullets say it is a powder, safe for pond fish, plants, pets, and animals, and that you do not need to turn off the pump or filters. They do not state a repeat interval. A question on the page asks whether it can be used in all seasons, and the page does not answer it. At about £0.91 per 1,000 litres of stated capacity the arithmetic looks cheap, and an unreviewed 25 kg tub is still a trade purchase. It belongs with a pond that is actually near that volume, not with a garden pond that would be overdosed by a loose scoop.
Pros
Cons
Best for: You run a pond near the stated 312,500 litres and you can measure the maker's 10 g per 125 litres.
Less suitable: You have a garden pond, or you need reviews on this ASIN before you spend about £284.28.
Specifications
Common questions
How much water does the 25 kg tub claim to treat?
The bullets say up to 312,500 litres.
Is there a star rating on the saved page?
No customer-review line was saved for this ASIN.
Does the listing say to switch the pump off?
No. The bullets say there is no need to turn off the pump or filters.
Twelve amazon.co.uk listings were scored from the stored pages, the review notes on those cards, and the maker dose pages named below. Shelf date 21 October 2026. There is no laboratory test. A litre figure counts only when that listing prints it, and a clashing spec cell is not the dose. The highest score is the top pick. The best value is the 4 kg tub, because the highest score is already the listing at about £19.47. The weights below sum to 100.
Amazon evidence
35%The star line on amazon.co.uk, then whether the reviews are about a pond. A large file of pond reviews scores higher than a high star line on a handful of ratings. Brine-shrimp reviews on the NT Labs listing are not treated as pond evidence.
Listing fit
30%A named job and a treat volume that agrees with the title score up. A form field or a volume field that clashes with the pack loses points. No litre figure, and a bundle that does not split the grams, score low.
Review outcome
25%Clears that match the stated job score up. Milky water followed by weed breaking down counts as a result. Points come off for a repeated report that nothing happened, a review file that is not about the pond, and a single newt-death report, which is one account and not a rate.
Value
10%Shelf price in GBP against a stated volume you can compare. The 4 kg tub is about £1.30 per 1,000 litres of stated capacity. The 25 kg tub is cheaper per stated litre and has no star line, so it does not take the value award.
Buy the growth and the litres before you buy the star line. A clear-out powder, a weekly course, a submerged-algae liquid and a preventative are not one job. Prices below are the saved amazon.co.uk listings of 21 October 2026. This is not a laboratory test.
Blanketweed treatment is a powder, liquid, or treatment-plus-prevention pack for hair-like filamentous algae, also called silkweed. It is not a treatment for pea-soup green water, duckweed, or a submerged plant you meant to keep. The Royal Horticultural Society separates those growths. A wrong diagnosis wastes the tub. Hair-like mats that you can twirl onto a stick are blanketweed. Green water is a different problem.
Pea-soup water and a hair-like mat are different growths. The RHS separates them. A blanketweed product does not clear green water, and the wrong tub wastes the dose.
On Blanket Answer, milky water for about five days is the expected stage, with the UV left on. A second scoop the same day misreads that stage. Clean the filter when the weed breaks down.
Pre-Vent is a follow-up every 10 to 14 days, not a clear-out for a thick mat. envii lists copper sulfate, and one review reports newt deaths. That report is one account, not a rate. A wildlife pond starts with a stick, shade, and barley straw.
Confirm the growth is hair-like blanketweed, not green water, duckweed, or a plant you meant to keep. The RHS tells you to twirl mats out, shade a third to a half of the surface, and use barley straw only to discourage new growth, at about 50 g per square metre. Leave pulled weed on the bank for 48 hours. Then calculate litres as length times width times average depth in metres, times 1,000. Match a stated treat volume that agrees with the title.
A clear-out and a preventative are different tubs. Blanket Answer is the clear-out. The maker uses 10 g per 125 litres, expects milky water for about five days, and leaves the UV on. Pre-Vent is every 10 to 14 days after that, best above 10°C, and the listing states no litres. The bundles do not split the grams. Stop Blanketweed is 30 g per 900 litres once a week for about three weeks, then monthly, and HSE No. 9647 is the maker's own statement for that product.
Three typical situations, each with a recommendation from this comparison.
An 800 g tub matches the maker's size table when the pond is under about 10,000 litres and the growth is hair-like blanketweed. Cloverleaf Blanket Answer 800 g is the top pick at about £19.47, with 4.5 from 4,486 reviews and a stated 10,000 litres. Expect milky water, then clean the filter.
A larger fish pond near the stated 50,000 litres needs more powder, and dying weed still sends a sludge pulse through the filter. Cloverleaf Blanket Answer 4 kg is the best value at about £64.99. The bullets say the pump can stay on. Plan to net weed and rinse the media.
Pre-Vent is not the tub for a solid mat. Cloverleaf Pre-Vent 800 g at about £21.79 is sold as a follow-up every 10 to 14 days, best above 10°C, after Blanket Answer. The page states no treat volume. It is 4.9 from 27 reviews, a small file beside the clear-out tub.
The top pick for a fish pond near a stated 10,000 litres is Cloverleaf Blanket Answer 800 g at about £19.47. The volume agrees on the bullets and the volume field, amazon.co.uk shows 4.5 from 4,486 reviews, and reviewers describe milky water then weed breaking down. The best value for a pond near a stated 50,000 litres is the 4 kg tub at about £64.99, about £1.30 per 1,000 litres of that capacity, with 4.6 from 111 reviews. Pre-Vent is a follow-up, not a clear-out. envii is a liquid for submerged strands and lists copper sulfate. None of these scores is a laboratory test.
Cloverleaf Blanket Answer 4 kg
About £1.30 per 1,000 litres of a stated 50,000 litres, at about £64.99, with the pump left on.
Editorial
Peter F. is the editor for DIY & tools at productscompared.co.uk. Comparisons in this category are checked against amazon.co.uk listings, review evidence and public UK sources.
Guides on what to look for, how to use a product, and common mistakes.
Public UK and maker sources from the research digest, used for this page in October 2026. Prices, stars and review counts are from the twelve amazon.co.uk listings captured on 21 October 2026. HSE No. 9647 is Evolution Aqua's own statement for Stop Blanketweed, not a certificate for this set. A safe-for-fish bullet is a listing claim.
This page contains affiliate links to amazon.co.uk (tag pcuk26-21). If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are from the saved listings. This is not a laboratory test.
