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How to use a pool algaecide

Updated: 16.10.2026

The bottle is on the patio, the water is dull green, and the listing only says the liquid is easy. Easy is how a Saturday dose becomes an overdose. This is how to use pool algaecide on a UK garden pool: brush, test, run the pump, and pour only what your own label says. The [algae killer pool comparison](/hub/pool-care/algae-killer-pool) names the six cards and does not rank them again here. Amazon links are affiliate links. A qualifying purchase can earn a commission at no extra cost to you. Products Compared did not time a pump. Where two Kleen Pool retailers disagree, the bottle in your hand wins.

The bottle is on the patio, the water is dull green, and the listing only says the liquid is easy. Easy is how a Saturday dose becomes an overdose.

Our top picks

KLEEN POOL Kleen Pool 1 litre algaecide

Best overall

KLEEN POOL Kleen Pool 1 litre algaecide
8.5/10about £17.95
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KLEEN POOL Kleen Pool long-life algaecide

Alternative

KLEEN POOL Kleen Pool long-life algaecide
6.6/10about £17.95
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Clearwater Clearwater 1 litre algaecide

Best value

Clearwater Clearwater 1 litre algaecide
8.2/10about £9.55
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On this page
  1. 1.Read the bottle before you open it
  2. 2.Brush, balance, circulate, then pour
  3. 3.Do not mix the bottle with shock
  4. 4.The next day is for the filter, not a double dose

Read the bottle before you open it

Clearwater’s bullets say the dose depends on volume and is written on the bottle. They print no millilitres. Do not borrow the Kleen Pool table and call it a Clearwater dose. If the print is hard to read, you still need that label. Kleen Pool shows the opposite gap. The Amazon title says 28,000 litres for the 1 litre size, while the millilitre table sits on trade pages that disagree. Pool & Spa Centre gives pH 7.2 to 7.6, chlorine at 1 to 3 mg/l, a pour near the inlets with the pump on, and 250 ml for 6,820 litres, 500 ml for 14,100 litres, or 1 litre for 28,200 litres. It says never to mix the liquid with other chemicals, including chlorine. World of Pools instead says raise chlorine to 6 ppm first, at the same pH. Those are different instructions. Use the bottle you bought. 28,000 and 28,200 litres are close, and neither figure is a reason to round a small pool up to a full bottle. Check the chemistry first on the buying guide.

Brush, balance, circulate, then pour

Brush the walls and floor first, so you are not dosing a film the water never reaches. If the pool is sanitised, test pH and free chlorine before you add anything. PWTAG, for managed pools, describes a free-chlorine rise to 5 mg/l overnight, a scrub, and a turnover check. That line is operator practice, not your algaecide dose. Each 0.5 rise in pH roughly doubles the chlorine required. Then run the pump. One Kleen Pool review of a 10,000 litre pool keeps the filter in the result: shock one day, the algaecide the next, pump on. Pour near the inlets, the label amount, with the water moving. Clearwater’s UK reviews that end clear also mention the filter. Pump off, and the dose sits in a corner. Frame-pool scale, including the 15 ft stories, is on the above-ground page.
KLEEN POOL Kleen Pool 1 litre algaecide

Testsieger

KLEEN POOL Kleen Pool 1 litre algaecide
8.5/10about £17.95
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Do not mix the bottle with shock

Pool & Spa Centre tells Kleen Pool buyers never to mix it with other chemicals, including chlorine. One jug, one product. The 10,000 litre review, shock one day and Kleen Pool the next, is a gap in time. It is not a licence to combine them in a bucket on a Saturday. If the label says shock first, finish that step and circulate before you dose. Follow the line on your Kleen Pool bottle. A Clearwater bottle was never that table. Quat-type and copper-type liquids are a separate choice. They do not go into the same pour.

The next day is for the filter, not a double dose

Clearwater reviews often describe little change for a day or two, then clearing if the filter keeps running. Heavy algae sometimes gets a second full dose later, after the first has had that chance. One UK review describes about 20 times a normal dose. Treat it as misuse, not as a tip for teatime when the water still looks green. Brush or remove what has settled. More liquid on top of that debris turns a green pool into a soup. If you have followed the label, run the filter, waited, and the water is still green, stop and re-test. A yellow film you will not drain, a stain, or a school, hotel or holiday-let pool is when a pool engineer takes over. No card states a swim-wait, so this page will not invent one.

Common questions

Do I add algaecide before or after chlorine?
Follow the bottle. Pool & Spa Centre says keep Kleen Pool’s chlorine at 1 to 3 mg/l and never mix the two in one container. World of Pools says raise chlorine to 6 ppm first. A UK review of a 10,000 litre pool used shock one day and the algaecide the next, with the filter on. That gap is the safe reading of a do-not-mix line. Clearwater’s card prints no chlorine sequence, so its bottle is the only order.
Can I mix them in one jug?
No. Pool & Spa Centre’s Kleen Pool line is never to mix it with other chemicals, including chlorine. Shock, circulate, then dose later if the label allows that order. The same refusal covers a quat-type liquid and a copper-type liquid tipped in together. One jug means one product.
How long should the pump run?
Long enough for the dose to travel and for the filter to catch what you brushed. The 10,000 litre Kleen Pool account leaves the pump on as part of the result. These cards state no minute count. Pump off after you pour, and the chemical stays in one place. If you cannot run a filter, pause. The bottle will not stir the pool.
When can children swim?
These cards state no swim-wait, so none is invented here. Read the bottle and keep children away from the neat liquid. A star rating is not a cleanliness certificate. If a child swallows algaecide or gets it in an eye, contact NHS 111 or 999. Green water is a treatment problem, not that call.
The Clearwater listing has no millilitres. Where is the dose?
On the bottle. The cards say the amount depends on volume and is printed on the pack. This guide will not supply a figure the bullets omit. Kleen Pool’s trade table, 250 ml for 6,820 litres up to 1 litre for 28,200 litres, belongs to that brand. It is not a Clearwater dose. If the Clearwater label is missing, leave the cap on.

Related pages

Copper or quat pool algaecide

Algae killer pool

Algaecide for above ground pool, matched to litres

Best pool algaecide UK: for the job, not the stars

Pool algaecide buying guide: water, job, chemistry

Sources

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.

  1. HSE: biocidal product types, including pool algaecides (retrieved 15 October 2026)
  2. Retained BPR Annex V, product type 2 (retrieved 15 October 2026)
  3. HSE: UK authorised biocidal products (retrieved 15 October 2026)
  4. HSE: swimming pools and HSG179 (retrieved 15 October 2026)
  5. PWTAG: typical pool problems (retrieved 15 October 2026)
  6. Pool & Spa Centre: Kleen Pool dose table (retrieved 15 October 2026)
  7. GTS Shop: Clearwater CH0006 active (retrieved 15 October 2026)