The best algae killers for pools, compared.
Top pick: Kleen Pool 1 litre algaecide – Highest score on a United Kingdom sample of 585 reviews, with 1,000 ml and a 28,000 litre title claim. Best value: Clearwater 1 litre algaecide – Lowest shelf price in the set at about £9.55, on a UK sample at 4.5 from 7,537 reviews. Updated 16.10.2026.
This algae killer pool comparison is for a biocide you add to a swimming pool, a spa, or a paddling pool. Buyers mostly have an above-ground pool, a hot tub, or a winter cover. They are choosing between a cheap liquid, a long-life swimming-pool dose, and a specialist powder. What counts is a named volume, a dose printed on the bottle, a United Kingdom review sample, and chlorine with a filter that is running. None of the six amazon.co.uk cards prints a GB number. Water, job, and label are set out in the [algae killer pool buying guide](/hub/pool-care/algae-killer-pool/sub/algae-killer-pool-buying-guide). It is not a fish-pond treatment and not an aquarium bottle. Pool algaecides sit in biocidal product type 2.
This algae killer pool comparison is for a biocide you add to a swimming pool, a spa, or a paddling pool. Buyers mostly have an above-ground pool, a hot tub, or a winter cover.
22 products checked · 56 hrs invested · +21,400 readers
Swipe for more
1-2 of 6
Personal buying guide
Question 1 of 2
What water are you treating?
The Kleen Pool 1 litre card costs £17.95, and the title says that litre treats 28,000 litres and lasts 6 months, with 4.6 from 585 United Kingdom reviews. Across the first 50 of those reviews, winter closing is the pattern that repeats: half a bottle in a 15 ft Intex in autumn, then clear water at spring opening, and one account of about eight months under a cover. A stated 10,000 litre pool went from bright green to clear after a chlorine shock the day before, with the pump left running. The Amazon card does not name an active ingredient, and a hot tub is only a question on the listing. A minority say a whole bottle did nothing or the pool went greener, and settled algae still needs sweeping off the floor. An older review mentions leakage from the bottle. Six months remains a title claim. It is the top pick here for a swimming pool you will lay up or keep circulating, up to that stated volume.
Pros
Cons
Best for: A garden swimming pool you will close for winter or keep on a pump, up to the 28,000 litre claim on the title.
Less suitable: A spa that needs a confirmed small dose, or a pool you intend to leave with the pump switched off.
Specifications
Common questions
How much water does the Kleen Pool title claim to treat?
The title says 28,000 litres, and that it lasts 6 months. The raw file has no millilitre dose table.
Does this card say it is for hot tubs?
A question on the listing asks about hot tubs. The bullets do not answer yes. The title says swimming pools.
What do UK reviewers use it for?
The first 50 UK reviews often describe a winter close, including half a bottle in a 15 ft Intex. A minority say a whole bottle did nothing or the pool went greener.
This second Kleen Pool listing is also £17.95, with bullets that say up to 6 months and up to 28,000 litres, and the star line is 4.6 from 147 reviews. The unit-count field says 1 count rather than millilitres, so the card never confirms a 1 litre bottle, even though the weight is 1 kg and the image URL matches the other Kleen Pool listing. All 25 United Kingdom reviews in the file were read. They include half a bottle every six months in about 14,000 litres, 250 ml in a 12 ft frame pool that one person found clear the next morning, and a 9 ft paddling pool kept clean. One buyer says they picked it because it is copper based, which is a review comment rather than a specification. Another says the water coloured after about three weeks. The manufacturer field reads Happy Hot Tubs and there is no GTIN, so you are looking at a shorter United Kingdom sample at the same cash price.
Pros
Cons
Best for: A second look at the long-life Kleen Pool claim when you already know the bottle size is missing from the card.
Less suitable: A first purchase, when the other Kleen Pool card states 1,000 ml at the same price and has 585 reviews.
Specifications
Common questions
Is the bottle size stated in millilitres?
No. The unit-count field says 1 count. The title does not say 1 litre. Weight is 1 kg. Bullets say it treats up to 28,000 litres.
Does the card name the active?
No. One UK review says the buyer chose it because it is copper based. That sentence is not a specification line.
How does this listing differ from the other Kleen Pool ASIN?
Same snapshot price, £17.95, and the same image URL. This card has 147 reviews, manufacturer Happy Hot Tubs, and no GTIN. B00AWP8FBE has 585 reviews, a 1,000 ml unit count and a GTIN.
Clearwater’s 6 litre listing, model CH0006-BDL-6L, is £47.99, with a unit count of 6,000 ml, a listed weight of 6.96 kg and a number-of-items field of 6. The bullets copy the 1 litre card: hot tubs, spas, paddling pools and larger above-ground pools, with the dose said to be on the bottle and no millilitre figure in the listing. The star line is 4.5 from 7,540, three reviews above the 1 litre card’s 7,537, and the first 50 texts repeat the same names, dates and opening lines. Read that figure as one shared sample. The raw file does not say whether the parcel is one container or six bottles. At £47.99 against a £9.55 litre, the pack makes sense only if you will use several litres and the price per litre beats buying the small bottle again. The shared text includes a leak, a pool still green after a week, and a twenty-times overdose, and those lines are not faults shown for this bundle on their own.
Pros
Cons
Best for: A household that already uses Clearwater CH0006 and expects to get through several litres in a season.
Less suitable: A first bottle, or anyone treating 7,540 as proof that this bundle was reviewed on its own.
Specifications
Common questions
Is this a different formula from the 1 litre Clearwater card?
The bullets match the 1 litre card. The model is CH0006-BDL-6L. The raw file does not print an active ingredient.
Are the 7,540 reviews about this 6 litre pack?
The star line says 4.5 from 7,540. The first 50 review texts in this raw file match the 1 litre file, including the same names and dates. Treat them as a shared sample.
Which pools does the card name?
Hot tubs, spas, paddling pools and larger above-ground pools, in the bullets. The dose figure is not in the listing.
The Clearwater CH0006 bottle is 1 litre of liquid at £9.55, and amazon.co.uk shows 4.5 from 7,537 reviews. The bullets name hot tubs, spas, paddling pools and larger above-ground pools, and they claim the liquid both prevents build-up and removes algae already there. In the first 50 United Kingdom reviews, light growth often clears once the filter is running, including a Bestway-type 10 ft pool and a hot-tub film. The listing does not print a millilitre dose or a treat volume, only that the figure is on the bottle and depends on how much water you have. Several buyers still reported a green pool after a week or after using the whole bottle, and one review describes a leak from a closed bottle. A separate review poured about 20 times a normal dose, which is misuse. It is the best value in this set when you want a cheap liquid for a small vessel and you will follow that label.
Pros
Cons
Best for: A hot tub, spa, paddling pool or modest above-ground pool, where you will dose from the bottle and keep the filter running.
Less suitable: A single winter pour aimed at about 28,000 litres, or a buyer who needs the active printed on the Amazon card.
Specifications
Common questions
Does the Clearwater 1 litre card name paddling pools?
Yes. The bullets say it is designed for hot tubs, spas, paddling pools and larger above-ground pools. They do not name a fish pond.
Is the dose printed in the Amazon listing?
No millilitre figure is in the raw file. The bullets say the dose is written on the bottle and depends on water volume.
What do the UK reviews repeat?
The first 50 UK reviews include easy pouring and pools that cleared when the filter ran. They also include a still-green pool after a week, a leaking bottle, and people who dosed far above the label.
Yellow Treat is a powder sold as a 2 pound container at £47.74, model YTC12EACH, and the star line on amazon.co.uk is 4.3 from 2,002 reviews. Every written review in this raw file is headed United States, so the average is not a sample of shoppers in the United Kingdom. Those US texts describe mustard or yellow algae, brushing, and more than one dose alongside shock before the water looked blue. One long US review says a single dose left a stain and then left the pool unable to hold a chlorine reading until it was drained. The weight field says 0.28 g and the unit count says 946.35 millilitre, which conflict with a 2 pound powder and are left as scraped. The title does not say swimming pool, and the card names neither an active nor a dose. It belongs with a yellow film you already recognise, where you can follow a label this UK listing does not print.
Pros
Cons
Best for: A pool with a yellow or mustard film, where you will follow a label this UK card does not print and you will keep the US comments separate from British evidence.
Less suitable: Ordinary green water in a UK garden pool, a paddling pool, or any choice that depends on a United Kingdom review sample.
Specifications
Common questions
Does the Amazon card name the active ingredient?
No. The benefits line says it controls algae and maintains clean sanitised water. Form is powder. There is no dose and no pool volume.
Are the reviews from the United Kingdom?
The reviews in this raw file are headed Reviewed in the United States. The star line is 4.3 from 2,002. Do not read that as a UK sample.
Why do the weight figures disagree?
The package size says 2 pound. The weight field says 0.28 g, and the unit count says 946.35 millilitre. Those conflicts are left as scraped. They are not corrected.
Relax model PLARCH097SR is listed at £39.95 with 2 litre in the title, and the raw file contains no customer-review line. The bullets say the product is for a cure rather than prevention, that it also stops algae returning, and that a change can show within 24 hours. No review in this capture supports those lines, and the card prints no active ingredient, no treat volume and no dose. The unit-count field says 1 count while the title says 2 litre, and the weight field is empty. A hot tub is a question on the listing rather than a confirmation in the bullets. Rank 54 in Algaecides is a sales position on that shelf. At £39.95 it costs more than either Kleen Pool card, and you have no buyer text to set beside the 24 hour bullet.
Pros
Cons
Best for: Only a buyer who already holds the bottle and can read a dose the Amazon card does not show.
Less suitable: Anyone choosing from United Kingdom reviews, a confirmed hot-tub use, or a stated winter-close volume, at a price of £39.95.
Specifications
Common questions
Does the listing have a star rating?
No customer-review line is in the raw file. The comparison does not invent a score.
What does the card claim it does?
Bullets say it is for a cure rather than prevention, that it also prevents return, and that improvement can show within 24 hours. There is no active ingredient and no dose table in the file.
Is a hot tub included?
A question asks about hot tubs. The bullets talk about a pool. They do not say yes to a hot tub.
Six algae killers on amazon.co.uk were compared from the stored listings, the United Kingdom or United States review samples, and the public sources listed below. There was no lab dose and no water test. The top pick is the highest star line with at least 100 reviews whose raw sample is marked United Kingdom. The best value is the lowest GBP price among those UK-sample listings at 4.3 or above with at least 100 reviews. Display scores add stars, the review sample, listing clarity, price fit, and faults. They are not a laboratory result.
Water body
20%The card has to name the vessel you actually have. A hot tub, spa, or paddling pool is a different scale from a swimming pool up to about 28,000 litres. A fish pond is out.
Job
20%Winter lay-up and season-long prevention are a different job from green water already in the pool, and both differ from a yellow or mustard film.
Named chemistry
15%A trade page may name an active the Amazon card does not print. If the card is silent, that silence stays in the score. These six cards do not show a GB BPR number.
Label dose
20%A treat volume or a millilitre dose you can read beats a title that only says the figure is on the bottle. Where retailer tables disagree, the bottle you hold wins.
UK review sample
15%Written reviews marked United Kingdom count. A shared sample, a United States file, or a card with no review line does not stand in for a British sample.
Price fit
10%On this snapshot a reviewed 1 litre bottle sits under £20. A multi-litre pack or a specialist powder near £40 to £50 has to earn that outlay.
Start with the water you have, then the job, then a dose you can read on the bottle. A United Kingdom review sample and a running filter come before a six-month title or a 24 hour bullet.
A pool algaecide in this comparison is a biocidal product for a swimming pool, a spa, or a paddling pool. It is not a fish-pond treatment and not an aquarium bottle. The six amazon.co.uk cards here are liquids or, in one case, a powder. None of them prints a GB BPR authorisation number. Pool algaecides sit in biocidal product type 2. Authorisation is per product. A number such as UK-2018-1126 belongs to a different copper range, so these six are not described here as HSE-approved.
The Clearwater 6 litre card shows 4.5 from 7,540. The written sample matches the 1 litre file, including the same names and dates. Read it as one shared sample.
Six months is a title claim on the Kleen Pool cards. Reviews that stayed clear still describe a cover, a shock, or a pump left running. Settled algae still needs sweeping.
Aquarium and pond products were left out of this set. Yellow Treat’s written reviews are marked United States. Do not cite them as a British sample, and do not multiply a dose in the first hour.
Name the vessel before you name a bottle. Clearwater’s card lists hot tubs, spas, paddling pools, and larger above-ground pools. Kleen Pool’s titles are swimming pools, with a claim up to about 28,000 litres. A 28,000 litre dose is the wrong scale for a spa. A fish pond is outside this comparison, and pond fish-safety lines do not apply. Then name the job. Winter lay-up and season-long prevention point at the Kleen Pool six-month claim.
Read the dose from the bottle you will hold. Amazon prints no millilitre figure for Clearwater, only that it depends on volume and is on the bottle. Kleen Pool’s 1 litre title states 28,000 litres and six months, with no dose table. Pool & Spa Centre runs from 250 ml for 6,820 litres to 1 litre for 28,200 litres, and says never mix it with chlorine. Another retailer says raise chlorine to 6 ppm first. The bottle wins that disagreement. The second Kleen Pool card never confirms a litre.
Three typical situations, each with a recommendation from this comparison.
You are laying up a garden swimming pool, or you will keep the pump circulating, up to the 28,000 litre claim on the title. Kleen Pool is about £17.95, with 4.6 from 585 United Kingdom reviews. Winter notes often describe half a bottle in a 15 ft Intex. Six months is a title claim, and the active is not on the card.
You need a small dose for a hot tub, spa, paddling pool, or modest above-ground pool, and you will read the bottle. Clearwater CH0006 is 1 litre at about £9.55, with 4.5 from 7,537 reviews. The listing does not print the millilitres. A 28,000 litre winter dose is the wrong scale.
Ordinary chlorine has not shifted a yellow or mustard film, and you will follow a label this UK card does not print. Yellow Treat is the only powder here, at about £47.74. The written reviews are marked United States. Keep those comments separate from British evidence.
In this set of six amazon.co.uk listings, the top pick is the Kleen Pool 1 litre algaecide. It has 4.6 from 585 United Kingdom reviews, a stated 1,000 ml, and a title claim of 28,000 litres for six months, at about £17.95. The active is not on the card, and six months is a title claim. The best value is the Clearwater 1 litre algaecide at about £9.55, for a hot tub, spa, paddling pool, or modest above-ground pool, if you dose from the bottle and keep the filter running. Yellow Treat is the mustard-film powder, and its written reviews are American. None of the six cards prints a GB BPR number.
Kleen Pool 1 litre algaecide
Highest score on a United Kingdom sample of 585 reviews, with 1,000 ml and a 28,000 litre title claim.
Clearwater 1 litre algaecide
Lowest shelf price in the set at about £9.55, on a UK sample at 4.5 from 7,537 reviews.
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 sources and the stored amazon.co.uk listings, from the research digest. Shelf prices and star lines are the UK figures in the display file, snapshot 15 October 2026.
This page contains affiliate links to amazon.co.uk. If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are from the UK listings in the 15 October 2026 snapshot. We did not dose these products or test the water.
