Why is my pool water green?
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Part of our pool and hot tub water guide
You pulled back the cover and the water has gone green. What do the UK pool bodies say is behind it, and what does your own tap have to do with it? You can check your own water with a 20-in-1 test strip.
Short answer: Algae, according to PWTAG, the Pool Water Treatment Advisory Group. It says algal growth may be green, dark green or red-brown, and links it to sunlight, poor circulation and a disinfectant residual that may be too low. Its fix for pool operators is raising free chlorine to 5 mg/l overnight and scrubbing.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
Why is my pool water green?
Most likely algae, and the sources link it to sunlight, weak circulation and too little disinfectant. The Pool Water Treatment Advisory Group (PWTAG) says in its guide to typical pool problems that algal growth may be green, dark green (almost black) or red-brown, and that algae are caused by the effects of sunlight and by poor hydraulics, mixing and turnover. It also lists a disinfectant residual that may be too low.
The Swimming Pool & Allied Trades Association (SPATA) is a pool trade body. Its water treatment factsheet (C12) says bacteria, viruses and other micro-organisms are carried in by bathers and environmental pollutants and can multiply in untreated water, so a chlorine or bromine disinfectant is used. For tubs and ponds, see our green hot tub water guide and green pond water guide.
| Factor | What the source says |
|---|---|
| Sunlight | PWTAG says UV (sunlight) causes algae and also breaks down free chlorine. For outdoor pools it suggests considering cyanuric acid dosing to prevent chlorine loss |
| Weak circulation or filtration | PWTAG lists poor hydraulics and turnover, and says reduced turnover, a dirty filter and lower flow can make free chlorine low and hard to maintain |
| Low disinfectant or high pH | PWTAG lists a low residual. Its cloudy water note says a pH that is too high means disinfection is inadequate, and that the lower the pH the more active hypochlorite is |
What does PWTAG say about swimming in green or cloudy water?
In the PWTAG and SPATA pages we read there is no statement on green water as such, and PWTAG's code of practice lists domestic and paddling pools as not covered, though it says many of its principles can be applied to them.
- The cloudy water note says you should be able to see clearly, from the pool surround, the detail of the bottom at the deepest point. If not, it describes an obvious hazard and says disinfection will be compromised, and that a pool failing its clarity checks should be closed immediately for investigation.
- The code lists potential drowning where cloudy water prevents surveillance of swimmers under the water.
- The code says disinfection needs water that is clear and free of suspended material, which may shelter microorganisms from the disinfectant.
Whether your own pool is fit to use is a question for your pool contractor, and we give no verdict.
Why does my own tap matter for a green pool?
The area figure is an average. Your tap fills your pool with whatever your own supply carries. PWTAG's code says mains water, especially from a river or reservoir, is likely to contain some or all of organic materials, lime and other alkalis added to prevent corrosion in the supply network, and phosphates, which it says are added to stop lead and copper dissolving from pipework but encourage algal growth in the pool.
PWTAG's notes also say excessive hardness, for example from calcium hypochlorite in an already hard supply, can be problematic, and its typical pool problems page lists water that is too hard as a possible reason for scaling on surfaces. Our how hard is my water guide explains how to find your area's hardness. A strip dipped in a glass of your fill water has a hardness pad, which in our reading gives a rough comparison with that figure (steps at 0, 25, 50, 100, 250 and 425, with the unit basis unconfirmed). If you fill from a private supply, PWTAG says source water should meet drinking water quality standards, and that this applies to private supplies too. See our well and borehole water guide.
“Your tap fills the pool with whatever your own supply carries, and an area figure is only an average, so I would dip a strip in a glass of your fill water before assuming the numbers apply to you. It takes a minute and gives you something of your own to compare against, rather than relying on a figure for the whole area.”
How do the sources say to clear green pool water?
By raising the free chlorine, scrubbing and checking circulation, according to PWTAG. In the sources we read there is no official home-pool method, so the table is PWTAG's advice for pool operators and we give no dose of our own.
| Problem (PWTAG, Typical Pool Problems) | What it says to do |
|---|---|
| Algal growth, discoloured walls and floor | Raise free chlorine to 5 mg/l overnight, scrub discoloured areas vigorously, vacuum or brush algae from surfaces, and check water distribution and turnover |
| Free chlorine low and difficult to maintain | Check the filter, flow rates, strainer and valves, and consider cyanuric acid dosing for an outdoor pool |
| pH difficult to lower | It names alkalinity that is too high, and says to increase acid dosing and/or dilute with fresh water |
Our pool pH guide covers the ranges.
How do you keep pool water clean and clear?
By keeping the disinfectant and pH in range, the filter and circulation working, and bather dirt out, according to PWTAG's public pool guidance.
- Disinfectant and pH. PWTAG's cloudy water note gives 7.0 to 7.4 as the recommended pH range for the free chlorine residual in pools.
- Filtration and circulation. The code calls effective filtration key to disinfection, and its turnover table gives 4 to 8 hours for domestic pools. Our paddling pool guide covers smaller pools.
- Bather pollution. The code says the chief sources of organic matter are bathers, through sweat, skin particles, mucus, urine and faeces, and that pre-swim hygiene can minimise it.
How can a test strip help with green pool water?
It indicates your free chlorine, total chlorine, pH and total alkalinity in 30 to 60 seconds, and free chlorine, pH and alkalinity are readings PWTAG's pages above refer to. Fill a clean glass or cup with pool water, dip one strip, wait 30 to 60 seconds and match the pads to the chart.
| Figure from the source | What the pad steps can show |
|---|---|
| Free chlorine 5 mg/l overnight for algae (PWTAG, pool operators) | The free chlorine steps are 0, 0.5, 1, 3, 5, 10 and 20 mg/l, so a step at 5 matches |
| pH 7.0 to 7.4 (PWTAG, cloudy water note) | The pH steps are 6.2, 6.8, 7.2, 7.6, 7.8 and 8.4. 7.2 is the only step inside that range, and the strip cannot tell 7.2 from 7.4 |
It has limits.
- There is no pad for cyanuric acid or phosphate, and it does not measure algae or clarity.
- Combined chlorine is total minus free, and at these steps that is only a rough comparison.
- We do not claim the rapid bacteria test for pool water, as it is for drinking water, and a strip result is not a legal sample.
For the method, see how to test your water at home. Scan the QR code on the colour chart, or open the free results checker, and the report opens in your browser with no app or account.
The area figure is an average, and your tap may differ because PWTAG's code says mains water, especially from a river or reservoir, is likely to contain some or all of organic materials, alkalis and phosphates. A low free chlorine step fits the low residual PWTAG lists, a pH step above 7.2 fits its high pH reason, and a high hardness step fits the too-hard water it lists for scaling, so a change between tests points to which one has moved. Best pack: 20 strips, enough for a test a day for a fortnight plus 6 re-tests while you work on the water (our arithmetic), £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your green pool water questions
How to keep pool water clean, how to keep pool water clean and clear, and how to keep swimming pool water clean
PWTAG points to disinfectant and pH in range, a working filter and pre-swim hygiene.
Is it safe to swim in green pool water?
In the PWTAG pages we read there is no statement on green water as such. Its cloudy water note says that if you cannot see the detail of the bottom at the deepest point, disinfection is compromised. Domestic pools are outside its code, so ask your pool contractor.
How to make pool water clear, how to keep pool water clear, and how to make swimming pool water clear
PWTAG's cloudy water note names low free chlorine, a failed circulation pump and a high pH as the three main causes of bacteriological cloudiness, and also lists algal growth.
How often is public swimming pool water changed?
In the PWTAG pages we read there is no interval for emptying a public pool. Its code says operators should replace pool water with fresh water as a regular part of treatment, at 30 litres per bather.
Why does pool water turn green, and why is my pool water green?
The sources point to algae, helped by sunlight, weak circulation and low disinfectant.
How to clear green pool water fast, how to fix green pool water, and how to get pool water clear from green
In the PWTAG and SPATA pages we read there is no method for a home pool and no figure for speed. PWTAG's operator step is raising free chlorine to 5 mg/l overnight.
Sources
- Pool Water Treatment Advisory Group, Typical Pool Problems (algal growth, low free chlorine, poor hydraulics, pH difficult to lower; undated; written for pool operators; read 29 October 2026).
- Pool Water Treatment Advisory Group, Code of Practice: The management and treatment of swimming pool water (July 2021, last updated July 2025; written for public pools; scope 1.8, hazards 3.1, mains water 3.2, source water 3.3, disinfection and bather pollution 3.4, dilution 3.6, turnover Table 2, pool essentials; we read the sections up to the start of section 4; read 29 October 2026).
- Pool Water Treatment Advisory Group, Cloudy Water (Technical Note 34, January 2026; clarity, closure, causes, pH range; written for pool operators; read 29 October 2026).
- Swimming Pool & Allied Trades Association (SPATA), Water Treatment (C12) (a trade body, so its statements are not official limits; the text we read covers disinfectants only; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, 20 strips £5.99, optional bacteria test +£5.00, free 2nd Class delivery on every order) and results portal. The strip-chart table and the list of limits are WaterKit's own reading of the colour chart steps.