Why Your Pool Turns Green & How to Fix It Fast

2025-01-12

There are few things more deflating than opening the curtains on a Saturday morning and finding the pool has turned green. It is one of the problems we are asked about most in Centurion and Pretoria, especially between November and March. The good news is that a green pool can almost always be recovered without draining it. This guide explains what turns a pool green, how to clear it yourself step by step, and when it is quicker and cheaper to call someone in.

What actually makes a pool turn green?

Algae spores are in every pool all the time. They arrive on the wind, in rain, on leaves and on swimming costumes. Normally the chlorine in the water kills them as fast as they land. A pool turns green when, for a few days, there is not enough working chlorine to keep up — and once algae gets a foothold, it multiplies quickly in warm water.

The usual reasons the chlorine falls behind:

Low free chlorine. Hot sun, heavy swimming and warm water all use chlorine up faster than most people expect.
High pH. When pH creeps above about 7.8, chlorine becomes much less effective. You can have a "normal" chlorine reading and still grow algae.
Storm water. A Highveld thunderstorm dilutes the chlorine and washes in dust, pollen, leaves and nutrients in one go.
Too few pump hours. Algae thrives in still water. Load-shedding, a tripped earth leakage or a timer that reset itself can leave the water standing for most of the day.
A tired filter. Old, clumped sand lets fine particles and algae straight back into the pool.
A scaled salt cell. In hard-water areas a salt chlorinator cell can stop producing chlorine while the control box still says it is running.
Food for algae. Garden fertiliser, leaves rotting on the floor and, in Pretoria in spring, jacaranda blossom all feed it.

Is it algae, or something else?

Look closely before you treat it. Bright or dark green water, with walls that feel slippery, means algae. Water that is clear but tinted green, with no slime at all, can be dissolved metal — copper, or iron from borehole water — and a heavy dose of chlorine can turn that into a stain on the surface. A yellow-green powder that brushes off easily in shady corners is usually mustard algae, which is more stubborn and needs more thorough brushing. If you are not sure, test before you add anything, or send us a photo.

How do I clear a green pool myself?

For a light-to-medium green pool, this is the sequence that works. It takes a few days, not a few hours.

Clear the debris. Scoop leaves off the floor and the surface, and empty the weir and pump baskets. Organic debris uses up chlorine you want working on the algae.
Test, then fix the pH first. Bring the pH down to around 7.2–7.4 with pool acid before you shock. Chlorine added to high-pH water is largely wasted.
Clean the filter. Backwash and rinse, so you start with a clean filter.
Shock the pool. Use a shock dose sized to your pool's litres — the product label gives it — and for green water expect to need two to three times the normal amount. Add it in the evening so the sun does not break it down before it works. Pre-dissolve granular chlorine in a bucket of pool water, never the other way round, and never in a bucket that has held acid.
Brush everything. Walls, floor, steps, behind the ladder and the corners around the weir. Brushing breaks up the algae so the chlorine can reach it.
Run the pump around the clock for the first day or two, and backwash whenever the pressure climbs.
Vacuum the dead algae to waste. When the water turns a cloudy grey-blue, the algae is dead and settling. Vacuum it with the valve on Waste so it leaves the pool instead of going back through the filter. A clarifier or flocculant can help it settle.
Re-test and re-balance. Once the water is clear, bring chlorine and pH back into range before anyone swims.

If the water is still green after two days of good filtering, the chlorine level was not held high enough for long enough. Test again and repeat the shock, rather than reaching for an algaecide on its own.

How long does a green pool take to clear?

A light green pool is often swimmable again after a couple of days of treatment. A mid-green pool usually needs several days of continuous filtering and more than one vacuum. A pool that has gone dark green or black after weeks of neglect can take up to a week, because the filter can only process so much dead algae before it needs backwashing again. Pump run-time is the biggest factor you control.

Do I need to drain a green pool?

Almost never. Draining wastes water that Gauteng is often short of, and an empty pool can be damaged by groundwater pressure, especially after summer rain. Fibreglass pools are particularly at risk. Partial draining only makes sense when testing shows the chemistry cannot be corrected — for example, a stabiliser level so high that chlorine can no longer do its job.

Why does my pool keep turning green?

If a pool goes green more than once a season, something is causing it, and more chlorine alone will not fix it. The usual culprits:

The pump is running too few hours, or not at the hours the timer says.
The filter sand is spent. A sand change and filter service fixes that.
The salt cell is scaled with calcium, or the salt level is low.
The pH is being left high, so the chlorine never works properly.
Nobody tests the water between mid-December and mid-January.

When should I call in a professional?

Call someone in when the pool is dark green, when you have shocked it twice without success, when the pump or filter is not working properly, or when you simply do not have the week it takes.

Our green pool recovery is priced by pool size: R1,200 for pools up to 30,000 litres, R1,500 for 30,000–50,000 litres, and R1,800 above that. It covers testing, clearing debris, correcting the pH, a shock dose sized to your pool, brushing, filtering, backwashing and vacuuming to waste, and a check of the pump, filter and chlorinator so we can tell you what caused it. Any equipment fault we find is quoted separately and only done with your go-ahead.

When you WhatsApp us, send a photo of the pool, your suburb and its rough size, and we will confirm the price and arrange a date. We work across Centurion, Pretoria and Midrand.

How do I stop it happening again?

Test weekly, and again after every big storm. Keep the pH in range. Run the pump long enough, at hours when you have power. Backwash when the gauge says so, and change the sand when it is spent. Clean a salt cell when you see white scale on its plates.

If you would rather not think about it at all, a weekly pool maintenance plan keeps the water tested and balanced every week — particularly through the December holidays, when most green pools happen.

Frequently asked questions

Can I swim in a green pool?

No. Green water means there is not enough working chlorine to control bacteria, and you cannot see the bottom, which is a real risk with children around. Wait until the water is clear and the readings are back in range.

Should I use algaecide or chlorine?

Chlorine does the killing. Algaecide is useful afterwards as a preventive, but on its own it will not clear a green pool.

Why did my pool turn cloudy after I shocked it?

That is usually dead algae and fine debris in suspension. Keep the pump running, backwash when the pressure rises, and vacuum to waste once it settles.

Why is my pool green when the chlorinator is on?

Usually the cell is scaled with calcium or the salt level is low. Clean or service the cell, check the salt, and shock the pool to catch up.

What causes a pool to go green overnight?

It is rarely overnight. The chlorine has usually been low for a few days; a hot spell, a thunderstorm and a pump that missed its hours is the combination we see most.

Talk to us

WhatsApp or call Crystal Reef on 081 577 5051, or email sales@crystalreef.co.za. For the wider routine, read the complete guide to pool maintenance in Centurion.