Green pool water is algae, and algae only takes hold once something else has already given way — usually free chlorine running too low for the heat and bather load, circulation that isn't reaching every corner, or pH sitting high enough to blunt whatever sanitizer is left. Work out which one failed first and the fix is normally a focused few days, not a drain-and-refill.
What Causes Green Water in a Swimming Pool?
Algae spores are in the air, in rain, on swimwear and on pool toys — they are always arriving in the water. A healthy pool destroys them faster than they can establish. Green water shows up when that balance tips, and it almost always comes down to one or more of these:
- Free chlorine too low for the load. Hot weather, direct sun and a busy pool all consume sanitizer faster than a routine dosing schedule expects.
- pH drifted high. Hypochlorous acid — the actual disinfecting form of chlorine — works markedly less well as pH rises, so a chlorine reading can look acceptable while the water is still under-disinfected.
- Poor circulation. Corners, steps, behind ladders and anywhere a return jet doesn't reach stay green even when the bulk of the water tests fine.
- A filter that isn't keeping up. A dirty filter or one running too few hours a day can't clear the dead algae and debris chlorine has already killed, so the water stays hazy even as the chlorine reading climbs back up.
- Phosphate and nutrient load. Fertiliser runoff, leaves and organic debris feed algae directly and make it come back even after a pool has been shocked clear once.
Why Does a Pool Turn Green So Quickly?
Because algae growth is exponential once it gets a foothold. A pool that looked fine two days ago can be visibly green today, particularly after a hot spell, a heavy rain that diluted the chlorine, or a week where nobody topped up the dosing. By the time algae is visible to the eye, the population in the water is already well past the point where a small routine dose will catch up with it.
How Do You Tell Which Cause Is Behind Your Green Pool?
A quick diagnostic pass saves a lot of guesswork before you start adding chemicals:
| Likely cause | What you'll notice | What to check first |
|---|---|---|
| Low free chlorine | Green tint appears fastest in the shallow end and near the steps | Free chlorine specifically, not just a total chlorine reading |
| High pH | Chlorine reading looks fine but algae keeps growing anyway | pH test, and how long it's been since it was last corrected |
| Poor circulation | Green patches in corners and low-flow areas, rest of the pool looks clearer | Return jet coverage and pump run time |
| Filter not keeping up | Water is green and hazy, not just tinted | Filter pressure gauge and last backwash or clean date |
| Phosphate or nutrient load | Green blooms keep coming back even after readings test normal | A phosphate test kit, and what's shedding into the water nearby |
How Do You Clear Green Pool Water, Step by Step?
Once you know roughly what let the algae in, the order matters more than the products:
- Correct pH first. Chlorine added at the wrong pH is partly wasted before it does any work.
- Shock the pool. A single large, calculated dose — breakpoint chlorination — overwhelms an established bloom in a way a routine top-up dose won't.
- Brush every surface. Algae colonies growing on walls and the floor are protected from the water chemistry until they're physically broken up.
- Run the filter continuously until the water clears, backwashing or cleaning the media as often as the pressure gauge calls for it.
- Vacuum dead algae to waste once it settles, rather than running it back through the filter.
- Retest free chlorine daily until it holds steady, then set a dosing plan matched to the current bather load and weather — not the one that worked in spring.
Can Nanobubble Treatment Stop a Pool Turning Green in the First Place?
This is where a technology like NanoSWIM's plays a supporting role rather than the lead one. Its nanobubble systems deliver oxygen or ozone as bubbles under roughly 200 nanometres — small enough that they stay suspended in the water for days instead of rising and bursting in seconds. The nanobubble technology page explains the mechanism in full, but the relevant part here is contact time: more time in contact with the water means more of the organic and nutrient load gets oxidised continuously, rather than only at the moment a dose is added.
Be clear about the limit, because this is a footnote to pool care, not a replacement for it: nanobubble treatment is installed inline in the existing circulation loop, after filtration, and it leaves no persistent residual of its own sitting in the basin. It does nothing to fix a pool that has already turned green — that still needs shocking, brushing and filtration exactly as above. What it changes is the steady-state load algae has to feed on between those events, which is a prevention argument, not a cure. It also doesn't replace correct filtration, sensible pH control or a competent operator; it reduces the load on all three, not the need for them. A Norwegian public pool has to keep meeting the residual and monitoring requirements set out on the standards and regulation page regardless of which treatment method sits in the plant room.
If you're weighing chlorine, UV, salt and ozone-based approaches against each other more broadly, the treatment comparison page sets out what each one actually changes and what it leaves for the operator to still manage.
Frequently Asked Questions
Does shocking a pool always clear green water?
Shocking clears green water when it's paired with brushing the algae off surfaces and running the filter continuously until the debris is gone — on its own, a shock dose kills the algae in suspension but leaves colonies on the walls and floor to regrow. NanoSWIM, a Bergen-based water treatment supplier, sees pools go green again within days when the filter side of the job gets skipped.
Can you swim in a pool while it's still green?
No — treat a green pool as closed until it has been shocked, brushed, filtered clear and the free chlorine and pH have both tested back within the normal range for your facility, following your local health authority's guidance. This is a water-chemistry and operational question, not a medical claim NanoSWIM is positioned to make.
Does nanobubble treatment stop pools turning green in the first place?
Nanobubble treatment from NanoSWIM reduces the organic and nutrient load algae feeds on by oxidising it continuously in the circulation loop, which helps prevent a bloom from starting, but it is a treatment-loop technology, not a substitute for filtration, brushing or pH control, and it will not clear a pool that has already gone green on its own.
In my own experience, I can usually tell a pool is drifting toward trouble before a single test strip does — the water loses that sharp, almost too-clear look days before anyone runs a reading, and by the time someone finally tests it, the numbers just confirm what the water already told you. I've walked into plant rooms where every log entry says the chemistry was within range right up until the morning it turned green overnight, and the answer was never hiding in the paperwork — it was in a dead corner behind a ladder that the return jets never quite reached. Once you've seen that a few times, you stop trusting the printout more than your own eyes.
If green water keeps coming back at your facility no matter how carefully you dose it, get in touch with NanoSWIM and we can look at where the load is actually coming from, against your own pool volume and circulation.