Cloudy Pool Water: Causes and How to Fix It
Cloudy water is one of the most common pool problems in Centurion and Pretoria, and one of the most misunderstood. It is rarely caused by one thing, and pouring in more chlorine or a bottle of clarifier without knowing why the water is cloudy often wastes money and time. This guide explains the usual causes, the order to check them in, and how to get the water clear again.
Before anything else, a safety point. If you cannot see the floor at the deep end, nobody should swim. Cloudy water hides a swimmer in trouble, especially a child, so keep the pool closed until you can see the main drain clearly.
The usual causes of cloudy pool water
Water turns cloudy when it carries tiny particles the filter is not removing, or when the chemistry itself is throwing material out of solution. The common causes are these.
Low free chlorine
Chlorine kills bacteria and algae and breaks down the organic material swimmers and the garden bring in. When free chlorine falls below about 1 ppm, that material builds up faster than it is destroyed, and the water loses its sparkle. On the Highveld this happens easily in summer: strong sun burns chlorine off, and a busy weekend of swimming uses up the rest.
High pH or high total alkalinity
When pH climbs above 7.6, chlorine becomes much less effective, and calcium in the water starts to come out of solution as fine particles. High total alkalinity (above about 120 ppm) makes pH drift upwards and resist correction. Together they are one of the most common causes of milky, dull water.
High calcium hardness
Much of our municipal water is hard, and every top-up adds more calcium while evaporation leaves it behind. Above about 400 ppm, especially with high pH, calcium can cloud the water and leave scale on tiles and equipment.
A dirty, channelled or under-run filter
The filter is what actually removes the particles. If it is overdue for a backwash, if the sand has clumped so that water cuts channels straight through it, or if the pump simply does not run long enough each day, the water will not clear however good the chemistry is.
Storm debris, dust and pollen
A Highveld thunderstorm can dump dust, leaves and pollen into the pool in a few minutes, and dilute the chlorine at the same time. Spring pollen and dust from building work nearby do the same thing more slowly.
Dust from new filter sand
Freshly changed sand carries fine dust. If the filter was not rinsed properly after a sand change, some of that dust goes back into the pool and the water can look hazy until it is backwashed and rinsed again.
Early algae
Algae often shows itself as cloudiness before it shows as green. If the water looks dull and faintly tinted, and the walls feel slippery when you brush them, algae is getting started.
Diagnose it in this order
Work through the causes in a set order. Each step either finds the problem or rules it out, and the early steps are the cheapest to fix.
- Test free chlorine and pH. Use a fresh test kit or fresh strips. Free chlorine should be 1–3 ppm and pH 7.2–7.6.
- Test total alkalinity. It should sit at 80–120 ppm. If it is high, pH will keep creeping up.
- Check the filter pressure gauge. Compare it with the reading just after a backwash. A reading roughly 50–70 kPa above that clean reading means the filter needs a backwash.
- Check the pump's run-time. Look at the timer. If load-shedding or a power cut has knocked it out, the pump may be running at night or for half the hours you think.
- Look closely at the walls and floor. A slippery feel, a green or yellow tinge in corners, or patches on the steps all point to algae.
- Test calcium hardness. If everything else is in range and the water is still dull, high calcium is a likely suspect.
Step-by-step: clearing a cloudy pool
Once you know the likely cause, the fix is usually straightforward.
- Get the water moving. Empty the weir and pump baskets and run the pump for longer than usual, ideally around the clock until the water clears.
- Correct alkalinity, then pH. Bring total alkalinity into range first, then adjust pH to 7.2–7.6. Chlorine works far better once pH is right.
- Raise free chlorine. Shock the pool with a chlorine product at the dose on the label for your pool's volume, in the evening so the sun does not burn it off straight away.
- Brush the whole pool. Walls, steps, corners and behind ladders. Brushing lifts settled material and algae into the water where chlorine and the filter can reach it.
- Backwash and rinse. Backwash when the gauge rises, then rinse, so the filter keeps catching fine particles. Our step-by-step guide on how to backwash a sand filter explains the valve positions.
- Give it time and retest. A cloudy pool clears gradually over a few days of continuous filtering. Test again daily and correct as you go.
Flocculant or clarifier?
These two products are often confused, but they work in opposite ways.
A clarifier gathers the very fine particles that slip through sand into slightly larger clumps the filter can catch. You keep the pump running as normal. It is gentle and slow, and it suits mild haze once the chemistry is right.
A flocculant makes the particles clump together and sink to the floor. The pump is switched off so the water is still, the material settles, and it is then vacuumed carefully to waste, bypassing the filter, using the waste setting on the multiport valve. You lose water in the process and will need to top up and rebalance. Flocculant suits heavy cloudiness where a clarifier would take too long. Never vacuum flocculated material back through the filter, because it will clog the sand.
Neither product fixes the underlying cause. If the chemistry is off or the filter is failing, the water will cloud again. Always follow the dosing instructions on the label.
When the filter is the problem
If the chemistry is in range, the pump runs long enough and the water still will not clear, look at the filter. Signs that the sand has had its day include pressure that climbs straight back up after a backwash, water that stays cloudy even after a clarifier, dirt blowing back into the pool from the returns, and sand that feels clumped or crusted when the filter is opened.
Worn sand cannot be fixed with chemicals. The answer is new media. Our sand change and filter service replaces the sand and checks the valve and laterals, and is priced by how many bags your filter takes, from R1,450. The bag count is normally on the filter's label.
When cloudy is turning green
If the water is going from dull to a green or yellow tint, you are dealing with algae rather than simple cloudiness, and it will get worse quickly in warm weather. Test and correct the chemistry, shock the pool, brush hard and run the pump continuously. Our article on why your pool turns green and how to fix it goes through the process in detail.
If you cannot see the floor, or the pool has gone properly green, a recovery is usually quicker and cheaper than weeks of trial and error. Our green pool recovery service is priced by pool size, from R1,200.
Keeping it from coming back
Most cloudy pools are the result of small gaps in routine: a few days without testing, a timer that went wrong after a power cut, a backwash that was skipped. Testing twice a week in summer, backwashing when the gauge says so, brushing weekly and checking the timer after any outage will prevent most of it. If you would rather hand that routine over, a weekly pool maintenance plan covers testing, brushing, backwashing and equipment checks on every visit.
Getting help
If you have worked through the steps and the water still will not clear, send us a WhatsApp on 081 577 5051 with your test readings, a photo of the pool and a photo of the filter gauge. That is often enough for us to tell you what is going on. If a visit is needed: A call-out to inspect or diagnose is R650. Deducted from your quote when you accept it.