Cloudy pool water: why it happens and how to clear it

Illustration: the same pool, cloudy.

The usual causes

1. Free chlorine is too low. Heavy use, hot weather or rain uses it up.

2. pH is too high. Above 7.8, chlorine works poorly and calcium can come out of the water as a haze.

3. The filter is dirty or not running long enough. Health guidance says the filter should turn over all the water in 6 to 8 hours.

4. The chlorinator cannot keep up. HealthyWA notes a salt pool can turn milky if it is overused and the chlorinator cannot make enough chlorine.

Step by step

1. Test free chlorine and pH.
2. Fix pH first, then chlorine.
3. Clean the filter and run the pump continuously.
4. Brush the walls and floor.
5. Retest the next day. Most cloudy pools clear within a day or two once the chlorine and filter are right; a pool that stays cloudy may need a clarifier or flocculant, used as the label says.

Maple's tip

If it went cloudy straight after you shocked it, give it 24 hours with the filter running before adding anything else. Shocking can leave a milky haze that the filter clears by itself.

Each cause in more detail

Low free chlorine. When chlorine runs low, tiny particles and early algae multiply faster than the filter can remove them, and the water turns dull and milky. This is the most common cause after hot weather, a pool party or heavy rain. Test free chlorine first; if it is low, raise it.

High pH. Above the recommended range, chlorine works much less effectively, and calcium in the water can come out of solution as a fine haze. Bring pH back into range, usually by adding acid in small doses.

High total alkalinity or calcium hardness. High alkalinity makes pH drift upwards and helps calcium scale form, and high calcium hardness can cloud the water, especially when warm. Test both; lowering them takes acid and sometimes partial draining and refilling.

A dirty, small or damaged filter. A filter that needs backwashing or cleaning, has worn media, a torn cartridge, or is too small for the pool lets fine particles pass straight back into the pool. Check the pressure gauge, backwash or clean, and look for damage.

Not running the pump long enough. If the water is not turned over through the filter often enough, particles build up. In hot weather and after heavy use, run the pump longer. See how long to run your pool pump.

Heavy use, rain and dust. Sunscreen, body oils, dust storms and rain bring in particles and use up chlorine quickly. A busy weekend can cloud a pool that was clear on Friday.

Early algae. Before a pool turns green, it often goes slightly cloudy and dull. If you see a faint green tinge or slimy walls, treat it as algae: see green pool.

Clarifier or flocculant?

Clarifier clumps very fine particles into slightly bigger ones the filter can catch. You add it, run the filter as normal, and the water slowly clears over a day or two. It suits mild cloudiness when the chemistry is right.

Flocculant (floc) clumps particles into heavy clusters that sink to the floor overnight with the pump off. You then vacuum the settled layer to waste, bypassing the filter. It works faster on very cloudy water but needs more effort and wastes water. Not all filters and multiport valves can vacuum to waste, so check yours first.

Neither fixes the underlying cause. Correct the chlorine, pH and filter first, then use a clarifier or floc if the water still needs help.

How long cloudy water takes to clear

With chemistry corrected and the filter clean and running, mild cloudiness often clears within a day or two. Heavy cloudiness can take several days, with backwashing or cartridge cleaning along the way as the filter catches what it removes. If nothing changes after a few days of good chemistry and filtering, take a water sample to a pool shop for a full test, including phosphates and metals.

Preventing cloudy water

• Test chlorine and pH at least a few times a week in summer, and after heavy use or rain.
• Keep the filter clean and the pump running long enough.
• Brush walls and floor weekly to stop particles and algae settling.
• Empty skimmer and pump baskets regularly.
• Shower before swimming to reduce oils and sunscreen in the water.
• Keep stabiliser in range on outdoor pools so chlorine lasts in the sun.

Is cloudy water safe to swim in?

Cloudy water is a warning sign. If you cannot see the floor at the deep end, do not swim: a swimmer in difficulty could not be seen, and cloudy water often means chlorine is too low to keep germs under control. Fix the water first.

Cloudy water after shocking

A pool often turns cloudy right after a large chlorine dose, as chlorine breaks down organic matter and kills algae and the filter has not yet caught the debris. Keep the pump running, brush the walls, backwash or clean the filter as the pressure rises, and it should clear within a day or two. If you used calcium hypochlorite, the cloudiness may also come from calcium, especially in hard water.

Cloudy water after rain

Heavy rain dilutes chemicals, washes dust and garden soil into the pool, and often lowers pH. After a storm, empty the skimmer basket, test and rebalance chlorine and pH, run the pump longer, and brush the pool. If garden beds or lawns drain into the pool, consider redirecting runoff, because it brings phosphates that feed algae.

Cloudy water in a new pool or after a refill

A newly filled pool can be cloudy from dust, plaster or fresh water with high calcium or metals. Start-up routines from the pool builder usually include frequent brushing, filtering and careful chemical additions. If the water came from a bore or a tank, ask a pool shop to test for metals, which can cloud or stain the water when chlorine is added.

Cloudy water with a salt chlorinator

In a salt pool, cloudiness often means the chlorinator is not making enough chlorine: low salt, a scaled cell, too-short pump hours or too-low output. High pH, common in salt pools, can also cloud the water. Check the salt level, clean the cell, raise the output or run time, and bring pH down.

When to call a pool professional

If the water is still cloudy after a week of correct chemistry, a clean filter and long pump hours, take a sample to a pool shop for a full test including phosphates, metals and total dissolved solids, or book a technician to check the filter and pump. A cracked filter lateral, a torn cartridge, a pump that is not moving enough water, or very high dissolved solids can all keep water cloudy despite good chemistry.

A cloudy water checklist

1. Test free chlorine and pH; correct them first.
2. Test total alkalinity and calcium hardness; correct if high.
3. Check and clean the filter.
4. Run the pump around the clock until clear.
5. Brush the pool daily.
6. Use a clarifier, or floc and vacuum to waste, if needed.
7. Retest daily until the water is clear and chlorine holds.

Understanding the test results

Free chlorine is the chlorine available to sanitise. If it is below the target for your pool, raise it first; cloudy water with low chlorine is a sanitising problem.

Combined chlorine (total chlorine minus free chlorine) is chlorine that has already reacted with sweat, urine and other contaminants. High combined chlorine gives a strong "chlorine" smell and dull water, and is cleared by shocking.

pH shows how acidic or alkaline the water is. Above the range, chlorine weakens and scale can cloud the water.

Total alkalinity buffers pH. Very high alkalinity pushes pH up and encourages scale.

Calcium hardness shows how much calcium is dissolved. Very high levels, especially in warm water with high pH, can cloud the water with fine scale.

Stabiliser protects chlorine from sunlight in outdoor pools; too little and chlorine disappears on sunny days, too much and chlorine works slowly.

A pool shop can run all of these in a few minutes from a sample taken at elbow depth.

Water clarity and the filter run

Clear water needs the filter to catch particles faster than they arrive. In a busy summer, with swimmers, sunscreen, dust and leaves, the filter has more to catch, so longer pump hours and more frequent cleaning keep the water clear. In winter, the load is lighter. If the pool is clear in winter and dull every summer, extend the pump time in summer and keep the filter cleaner, rather than adding more chemicals.

Preventing cloudiness after parties

Before a pool party, raise chlorine to the top of the range, check pH, clean the filter, and run the pump through the event and for some hours afterwards. Ask swimmers to rinse off before getting in. The next morning, test and rebalance, brush the pool, and backwash or clean the filter if the pressure has risen.

Cloudy versus coloured water

Cloudy water is milky or hazy. Coloured water is different: green usually means algae, brown or rusty water can mean iron, and blue-green tinges can mean copper, especially after adding chlorine to bore or tank water. Metals need a metal sequestrant or removal product rather than more chlorine; ask a pool shop to test for them before treating.

Cloudy spa water

Spas are small and hot, with high bather loads for their size, so they cloud quickly. Keep sanitiser higher than in a pool, drain and refill as the spa maker recommends, clean the spa filter often, and use a spa-specific clarifier if needed. A cover keeps debris out and heat in.

Our short answer

Test and fix chlorine and pH first, clean the filter, run the pump continuously until clear, brush daily, and use a clarifier or floc only after the chemistry and filter are right. If it stays cloudy after a week, get a full water test.

Keeping water sparkling

Clear water comes from routine: test a few times a week in summer, keep chlorine steady, keep pH in the middle of its range, clean baskets and the filter on schedule, brush weekly, run the pump long enough, and cover the pool when you can. Most cloudy pools come from a break in that routine during a hot week, a party or a holiday, so a little extra attention at those times prevents most problems.

Cloudy water and pool covers

A pool kept under a cover stays cleaner, but covered water can still go cloudy if chlorine falls, because the cover also cuts the sunlight that would otherwise burn chlorine off and owners may test less often. Keep testing a covered pool on schedule, and take the cover off to let the water breathe after shocking. Clean the cover itself now and then, because dirt on its underside ends up in the water.

How Maple works it out

Maple's answers follow the order a pool professional uses: sanitiser first, then balance, then filtration and circulation, then clarifying products. Each step removes a likely cause before moving on.

Cloudy water and swimmer load

The more people swim, the more sweat, sunscreen, cosmetics and other contaminants enter the water, and the faster chlorine is used up. A pool that stays clear with two swimmers can cloud with ten. On busy days, test before and after, keep chlorine at the higher end of the range, run the pump longer, and ask swimmers to shower first. A pool shower or a hose near the gate makes that easy.

Brushing helps clear cloudy water

Brushing lifts fine particles and early algae off walls and the floor into the water, where the filter can catch them. While clearing cloudy water, brush the whole pool once a day, starting at the shallow end and working towards the main drain or the deep end, then let the pump and filter do their work. It speeds clearing noticeably, especially in pebble pools.

Common questions