Green pool recovery in the UK: from algae to crystal clear in 5 days

A green pool is not a mystery. It is algae plus a chlorine deficit, and in the UK it usually arrives one of two ways: you pull the cover off in April and find pea soup underneath, or a rare hot week in July outruns your chlorine feed. The fix is the same either way. No magic powder, no clarifier miracle — just chlorine held at the right level, daily brushing, and pump runtime.

Day 0: assess and balance

Before you spend a pound on chemicals, test the water. The two numbers that decide your fight are stabiliser (cyanuric acid, or CYA) and pH.

Stabiliser creep is the classic British problem. Most UK garden pools run on multifunction chlorine tablets, and every tablet adds stabiliser that never leaves the water. If CYA reads over 60 ppm (mg/L — same thing), drain a third of the pool and refill before you start. You cannot win against algae if you cannot get chlorine to its shock level.

If your pool is indoors or has never seen stabilised tablets, CYA may be near zero. That is fine — your shock target is simply lower (see the table below).

If pH is over 7.6, bring it down to 7.2. Pool shops sell hydrochloric acid or dry acid (sodium bisulphate) for exactly this job, and chlorine kills far faster at slightly acidic pH.

Finally, clean the filter properly. A dirty filter will choke on dead algae within 12 hours.

Day 1: SLAM (shock level and maintain)

SLAM means raising free chlorine (FC) to ten times your CYA level and keeping it there. So 30 ppm CYA = 30 ppm free chlorine target.

Doses assume 14% sodium hypochlorite, the usual UK trade strength — check the drum label and scale for your own pool's volume.

Add the chlorine in the evening. Brush every wall and the floor. Run the pump 24 hours a day until this is over.

Day 2: retest and recharge

Test free chlorine in the morning. It will have dropped a long way — that is the algae dying. Bring FC back up to the shock target, brush again, and keep the pump running.

The water will turn from green to grey. Grey is dead algae. You are winning.

Days 3 to 4: clean the filter, keep brushing

Backwash or rinse the filter every 12 to 24 hours — it is doing the heaviest work of its life. The water will go cloudy and milky before it clears. Hold chlorine at shock level. Keep brushing.

Day 5: the overnight chlorine loss test

When you think you are done, prove it. At sunset, record free chlorine. Before sunrise, test again. If you lost less than 1 ppm overnight and the water is clear, the algae is dead. Let chlorine drift down to 1–3 ppm and resume normal care — even a British summer has enough UV to bring it down within a few days.

If you lost more than 1 ppm, hold shock level for another day and retest.

What about algaecide

Polyquat 60 algaecide is a useful preventive when you open the pool in spring. It is not a fix for an active green pool — poured into a swamp it mostly wastes money and can cause foaming. Save it for after recovery.

When to call a professional

If the pool will not respond after five days of correct SLAM, you may be dealing with mustard algae, black algae, or phosphates feeding new growth as fast as you kill it. A pool engineer can identify and treat the problem — get a couple of quotes, because a full recovery usually means several call-outs.

Green pool FAQs

Will algaecide alone clear a green pool? No. Algaecide is a preventive. Once the pool is green you need chlorine at shock level for several days.

How much chlorine will I need? For a 50,000 litre pool with moderate algae, plan on 30 to 60 litres of liquid chlorine across the five days. Price it at your local pool or spa shop before you start — buying by the 20 litre drum is far cheaper than small bottles.

Can I swim before it is fully clear? No. Cloudy water hides the bottom, which is a drowning hazard, and shock-level chlorine is rough on skin and swimwear. Wait for full clarity and FC below 5 ppm.

Do I need to drain the pool? Only if CYA is over 60 ppm, or the pool is such a swamp that chlorine would cost more than a refill. Most green pools recover without draining.

Why did it turn green overnight? Free chlorine hit zero. Common UK causes: the tablet feeder ran empty during a warm week, a storm diluted everything, or high stabiliser quietly neutralised your chlorine. Find the cause or it will happen again.

A clear pool is worth something in the UK

Private pool hire by the hour has arrived in Britain, and a sparkling pool is the whole product. Pool Rental Near Me lets you list your pool free, and founding hosts pay 0% host fees, forever — you keep 100% of your price. Prices on the site currently display in US dollars, and hosts are paid out in pounds via Stripe. For context from the more mature US market: top backyard pools in Southern California earn $800–$1,000 in a good summer week. That is US data, not a UK promise, but it shows what warm-week demand looks like.

Start with our guide to renting out your pool in the UK, see how hourly hire works in London, and once your water passes the overnight test, List your pool.