Pool water has turned green or cloudy

A green or hazy pool looks alarming, but it's almost always a chemistry problem you can reverse in a day or two — most often the chlorine simply ran low and let algae move in.

tuneWhat are you noticing?

Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.

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Start here

In Canada, start with the sanitizer. Dip a test strip: if the free chlorine reads low or zero, that's almost certainly why the water went green — algae take hold fast once chlorine drops. The fix is to shock the pool back to the label dose and keep the filter running. Because shocking means handling chlorine, read the safety step first: never mix it with other chemicals, and always add it to water, not water to it.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

Dip a fresh test strip (or use a drop kit) in elbow-deep water away from the return jets, then compare the free-chlorine pad to the chart on the bottle.
A free-chlorine reading below about 1 ppm — or a pad that barely changes colour — means the sanitizer has bottomed out and algae have the upper hand.
Read the pH on the same strip: if it's well above 7.6, chlorine is working at a fraction of its strength even when it's present, which points you at balancing as well as shocking.

buildtest-strip schedule5 min

handymanHow to fix it

report Handle the shock safely — never mix chemicalsPool shock is safe when handled right and dangerous when it isn't. Never mix it with any other chemical — combining chlorine with acid releases toxic chlorine gas. Always add the shock to the water, never water to the product, and never one product on top of another. Wear eye protection, work in the open air, keep it in its original labelled container, and keep kids and pets off the deck while you dose. If any of this makes you uneasy, a pool company can shock and balance the water for you.

Test and balance pH first, then add pool shock at the label dose for your pool volume, running the pump and filter continuously until the water clears; re-test and re-dose the next day if needed.

Read the shock product label and work out the dose for your pool's volume in litres — under-dosing barely moves the needle on an algae bloom.
With the pump running, add the shock to the water exactly as the label says (many granular shocks are pre-dissolved in a bucket of pool water first). Never add water to the product, and never combine it with any other chemical.
Run the filter continuously (24 hours is common after a shock) and brush the walls and floor, keeping everyone out of the pool until the free chlorine falls back to a safe swimming range and the water is clear.
Re-test the next day; a stubborn bloom may need a second dose. If it still won't clear, look at phosphates or call a pool professional.

schedule60 min savings$15–60

Pool shock (chlorine)Calcium hypochlorite or sodium dichlor granular shock; match to your sanitizer type and dose to your pool volume in litres

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Homeowner pool water chemistry (testing, shocking, balancing) is permitted across Canada; there is no licensing requirement — the constraint is safe chemical handling and storage per the product label.

report Balancing chemicals: handle them one at a timeAdjusting pH means handling acid (pH down) or soda ash (pH up). Never mix pool chemicals with each other — acid and chlorine together make toxic gas. Add the chemical to the water, never water to the chemical, add only one product at a time and let it circulate before the next, and keep kids and pets away while you dose. Wear eye protection and store everything in its labelled container. Not comfortable? A pool company can balance the water for you.

report Shocking a bloom is chemical handlingClearing an algae bloom means a heavy shock dose. Never mix the shock with any other chemical (chlorine plus acid releases toxic gas), always add the shock to the water and never water to it, and dose only one product at a time. Wear eye protection, work in open air, keep the product in its labelled container, and keep everyone off the deck while you dose. If you'd rather not, a pool company can clear the bloom for you.

handymanHow to fix it

warning Release the filter pressure before you open itA pool filter runs under pressure. Opening the lid or clamp without shutting the pump off and bleeding the air-relief valve can launch the lid and cause injury. Shut the pump off, open the relief valve until the pressure gauge reads zero, then open the filter.

Clean or backwash the filter, then increase the daily pump run time so the whole pool turns over at least once or twice a day.

Check the filter's pressure gauge: a reading 8–10 psi above its clean baseline means it's time to backwash (sand/DE) or rinse the cartridge.
Turn the pump OFF and open the air-relief valve to bleed pressure before you open the filter — an unrelieved filter can blow its lid off under pressure.
Backwash or clean the element per your filter type, reseat it, and restart the pump.
Set the pump to run longer — many pools need 8–12 hours a day in swim season to keep the water clear.

schedule45 min savings$0–40

report Phosphate and metal treatments are chemicals tooA phosphate remover or metal sequestrant is a chemical you add to the water. Never combine it with any other product, always add it to the water rather than water to it, follow the label dose, and keep kids and pets away while you treat. Store it in its original labelled container. If you're unsure, a pool company can test and treat the water for you.

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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.