Why is my pool water cloudy?
Cloudy or milky pool water usually means low chlorine, poor filtration or water out of balance. How to find the cause and clear it.
Updated 18 September 2026 · 4 min read
Start with a test
Cloudy water is a symptom, not a cause, so test before you add anything. Run a strip through the app and look at free chlorine, pH, alkalinity and hardness. The answer is usually one (or more) of the causes below.
Common causes
- Low free chlorine, letting bacteria and early algae cloud the water.
- High pH or alkalinity, which makes calcium drop out of the water as a fine haze.
- Very hard water, especially combined with high pH.
- Poor filtration - the pump not running long enough, or a dirty or worn filter.
- A burst of use - a pool party, hot weather, pollen or suncream.
- Dead algae after shocking, which leaves the water grey or milky for a day or two.
How to clear it, step by step
- Skim, brush the walls and floor, and empty the skimmer and pump baskets.
- Correct alkalinity to 80-120 ppm, then pH to 7.2-7.6. Wait an hour between products and never mix them.
- If free chlorine is low or combined chlorine is high, shock in the evening with the pump running, following the dose in the app.
- Run the pump 24 hours a day until the water clears.
- Clean the filter: backwash a sand filter until the sight glass runs clear, or hose down a cartridge filter.
- After 24-48 hours, if it is still hazy, a clarifier helps the filter catch the finest particles. For very cloudy water, a flocculant settles the haze to the floor so you can vacuum it to waste.
How long it takes
Most cloudy pools clear within one to three days once chlorine and balance are right and the filter runs continuously. Keep testing daily and do not swim until free chlorine is back in range and you can clearly see the floor at the deep end.
Preventing cloudy water
Test two or three times a week, keep chlorine and pH in range, run the pump for at least 8 hours a day in summer, and clean the filter regularly. Shock after heavy use.
When to call a professional
If the water stays cloudy after a week of good readings and continuous filtration, your filter media may be worn out or the pump undersized. A professional can check your equipment.
Frequently asked questions
Can I swim in cloudy water?
No - if you cannot clearly see the floor at the deep end, it is not safe. Cloudy water may also mean chlorine is too low.
Why is my pool cloudy after shocking?
Shock kills algae and bacteria, which can leave a haze. Keep the filter running and it should clear within a day or two.
How long should my pool pump run?
At least 8 hours a day in summer, and 24 hours a day while clearing cloudy or green water.
Products that help
Available from Deep Blue Pool Supplies, with fast UK delivery.