Green pool water: what to do before a second bottle

You pulled the cover back and the frame pool is green. The strip from breakfast looked acceptable, the pump ran while people were in, and you are holding a brush and a bottle you have not opened. Green pool water in a UK garden is usually algae, after sunlight and a circulation run that stopped too soon.

The first hour is brush, pump, and a fresh test. Algaecide comes after that, at the dose on the bottle. Emptying is the honest answer when you cannot move the water you have just stirred up.

Products Compared read six algaecide listings on amazon.co.uk on 15 October 2026, with the UK review samples, PWTAG's typical pool problems page, and the Health and Safety Executive's swimming-pool guidance. There was no pool trial. Prices are GBP from that snapshot. A qualifying purchase through our links can earn us a commission at no extra cost to you.

In short

Green pool water is usually algae after sun and a short pump run, and a morning test strip does not describe the afternoon. Brush, keep the filter moving, and raise chlorine before you open a second bottle. Stay out of the water while it is green, leave pond treatments on their shelf, and treat two days of colour as a reason to check the pump, rather than as a timer that forces you to empty.

What green pool water is telling you

A green swimming pool overnight means algae has had light and too little moving, sanitised water. PWTAG, writing for managed pools, says algae may be green, dark green, or red-brown, and it names sunlight, poor turnover, and a low free-chlorine residual. A frame pool meets that in a hot spell: shallow water, the cover off at noon, and a cartridge pump switched off when swimming stops.

A cloudy green pool is the same growth once it is through the water, not only on the liner. Cloudiness tells you the filter has a job, because the film you brush off has to go somewhere. None of the six Amazon cards prints the active ingredient, so that name has to come from the bottle.

The pool can be green but chlorine looks fine when the strip was read at breakfast. Outdoor water loses chlorine in sun. PWTAG notes that each 0.5 rise in pH roughly doubles the chlorine a managed pool needs, so a fine chlorine square beside a high pH is weaker than it looks. Test again after the pump has run, and treat one morning reading as a snapshot of that minute. The Health and Safety Executive's swimming-pool pages are for public-pool operators. A school, hotel, or holiday-let pool is the operator's job.

Brush and circulate before a second bottle

Brush a green pool, and keep the water moving, before a second bottle. Film on the wall never meets a dose poured into the middle. Use a pool brush on the walls and the floor, including corners a cartridge inlet rarely washes, with the pump on. PWTAG's managed-pool action is to scrub, hold free chlorine up, and check that water is turning over.

1. Keep everyone out, including children, until the water is no longer green and the pack you used gives a re-entry line you can read.
2. Brush the walls and the floor so the film is in the water, and let the filter take it.
3. Confirm water is returning to the pool. A clicked switch is not circulation.
4. Test pH and free chlorine again once the water has been moving.
5. Raise chlorine on its own, then add algaecide at the amount printed on your bottle.

The limit is the pump. A choked cartridge, or no filter at all, turns brushing into greener soup. Frame-pool volumes for that fork are on the above-ground algaecide page. Run the pump through brushing and dosing, and overnight only if the pool maker allows that duty. PWTAG holds the raised chlorine overnight in a managed pool, then checks turnover.

Chlorine first, then algaecide for green water

Chlorine is the sanitiser. Algaecide for green water is the extra. PWTAG says raise free chlorine to 5 mg/l overnight, scrub, and check circulation, as managed-pool practice rather than a garden-bottle dose. A UK Kleen Pool review describes a 10,000 litre pool that cleared after a shock one day and the dose the next. Keep the two products out of one jug. Shock sits with the chlorine tablets comparison, and the pour order is on the how to use a pool algaecide page. Pool and Spa Centre says never to mix Kleen Pool with chlorine. Another retailer says 6 ppm first, so the bottle in your hand wins.

Clearwater CH0006 was £9.55 for 1 litre on 15 October 2026, at 4.5 from 7,537 reviews. A same-day snippet showed £12.99, so use the live price. The card names above-ground pools, the dose is on the bottle, and the remove-algae line is for a bloom already in the water. It is the best value on price among the UK-sample cards at 4.3 or above with at least 100 reviews.

Kleen Pool 1 litre is the top pick at 4.6 from 585 UK reviews, £17.95. The title's 28,000 litres and six months are protection, not a clearance time. The trade table runs from 250 ml for 6,820 litres to 1 litre for 28,200 litres. Twenty times the Clearwater label, in one UK review, is misuse, so use a second full label dose with the filter on. A swallow or an eye splash means NHS 111, or 999 in an emergency.

When the pool is still green after algaecide

A pool still green after algaecide needs a check of the pump, the chlorine, and the dose you already used. Clearwater's UK sample includes a pool still green after a week. Kleen Pool's includes a whole bottle that did nothing, or water that went greener. Other reviews in those files cleared when the filter stayed on. A bottle fails with the pump off, with chlorine never raised, or with a 28,000 litre dose in a much smaller pool. Heavy algae can also need a second label dose, and the written review does not always say which.

Confirm three things before you buy again. Water has to be returning, so clean a choked cartridge. Shock and algaecide have to have stayed apart, as in that 10,000 litre review. Test chlorine again the next day, in the sun. If one of those is wrong, another £9.55 or £17.95 repeats the miss. If all three are right and the water is still green after a proper filter run, a second label dose is the step some heavy-algae reviews describe. A different product poured on the same day is not that step.

The six-month title is a protection claim. One UK review describes half a bottle in a 15 ft Intex at closing time, which is one autumn pool, not a July formula. None of the six cards prints an authorisation number, so an Amazon bullet is not an HSE approval. A yellow or mustard film is a different problem, and this green-water sequence will not settle it.

When emptying is the honest answer

Empty when you cannot circulate, or when the algae you have brushed is too thick for the filter you own. A bottle does not catch debris. Nothing in these sources says that 48 hours of green water means you must dump. The 10,000 litre Kleen Pool review cleared without an empty: shock, then a dose, filter on. With no pump, a larger dose leaves the same green water.

Stay out while the water is green, including children, and including a clearer patch by the steps. None of the six cards states a wait time for children. If the amazon.co.uk listing omits a re-entry line, that gap is not permission to swim the same afternoon. Read the pack after you treat. If the line is missing, wait, test again, and ask the manufacturer.

Pond and aquarium bottles were removed from this comparison. A fish pond is not a swimming pool, and a pond treatment is not a faster route when the first pool dose has disappointed you. The six cards in scope, at 15 October 2026 prices, are on the pool algaecide comparison. Buy from that table once the pump is moving water.