Why pool water turns cloudy and how to make it clear again
Water turns cloudy when tiny particles stay suspended instead of being caught by the filter. The usual culprits are filtration, water chemistry (pH, chlorine, hardness) or dust. Find the cause first, then add a product.
Diagnosis comes first
Before you add anything, test free chlorine, pH, alkalinity and, if you can, hardness. Also look at the filter pressure gauge and check that the pump runs for as many hours as it should. The table shows the most common scenarios.
| What you see | Likely cause | What to do |
|---|---|---|
| Dull water, low chlorine, slippery walls | Early growth of algae or bacteria | Brushing and shock chlorination |
| Milky water, high pH and hardness | Calcium coming out of solution | Lower the pH, add a scale inhibitor |
| Cloudiness after rain, dust or pollen | Very fine particles | Clean the filter, use a clarifier |
| Correct chemistry, high pressure on the gauge | Dirty filter | Backwash |
| Correct chemistry, normal pressure | Too few running hours or worn-out filter media | More hours, check the filter |
| Cloudiness after a pool party | Sunscreen, sweat, chlorine used up | Shock chlorination, continuous filtration |
Filtration: the most common cause
The filter is what makes the water clear; the chemicals only help it. Check, in this order:
- Running hours: in hot weather circulation needs more hours. Until the water clears, leave the pump running continuously.
- Pressure: if it has risen above the clean-filter pressure, backwash the filter.
- Filter media: sand that has clumped or aged no longer holds fine particles; see changing the filter sand. Cartridges are rinsed with a hose and replaced when worn.
- Size: a filter too small for the pool volume will let the water go cloudy every time it gets hot; the selection criteria are in our article on the pool filter.
Two more signs: if you see fine sand on the floor in front of the return inlets, an internal part of the filter may have broken and be letting the media through into the pool. And if the water clouds up briefly right after backwashing, remember that a backwash always needs a short rinse afterwards; otherwise the first minutes of running send dirt back into the water.
Water chemistry and dust
With pH above 7.6, chlorine cannot keep up with organic matter and, at the same time, calcium tends to precipitate. If your water is hard, keep the pH close to 7.2 and add a scale inhibitor. After Saharan dust, hail or heavy rain, the water fills with particles so fine that they pass through the filter; the steps for this case are in our article on the pool after a storm.
Clarifier or flocculant
When the chemistry is right and the filter is clean but the cloudiness persists, one of these two products helps. They work differently, though.
| Clarifier | Flocculant | |
|---|---|---|
| How it works | Binds fine particles together so the filter can hold them | Binds them into flakes that settle on the floor |
| Pump | Runs normally | Stops for several hours, usually overnight |
| Afterwards | Clean the filter when the pressure rises | Careful vacuuming with the valve set to waste, then top up the water |
| Speed | One to two days | From the next day |
| Best for | Light, persistent cloudiness | Heavy cloudiness, mainly with a sand filter |
Flocculant is not suitable for a cartridge filter with no waste setting, because the flakes clog it immediately. In a liner pool, vacuum slowly, without dragging the vacuum head hard across the floor.
Questions
Can I swim in cloudy water?
It is not a good idea. Cloudiness often means the chlorine is not working properly, and in cloudy water you cannot see the floor, which is dangerous, especially for children.
The water went cloudy right after a shock chlorination. Is that normal?
Often, yes, especially with calcium hypochlorite in hard water or when algae are dying. Let the filter run for one to two days and clean it when the pressure rises.
See also
Pool filter
Sand, glass media or cartridge pool filters: how each one works, how to choose the size and what the pressure…
Green water
Green pool water: why it turns green, how to bring it back step by step with brushing, shock chlorination and…
Water hardness
Pool water hardness: what calcium does, why scale forms at the waterline, what to expect on islands and with…