Quick answer
Remove debris and brush every surface. Test pH, total alkalinity and stabilizer (CYA), bring pH into range, then raise free chlorine to the shock level appropriate for your CYA and run the pump continuously. Vacuum dead algae to waste, clean the filter, and do not swim until the water is clear and chlorine holds overnight.
What you're seeing
- Water is tinted green, blue-green or pea-soup cloudy.
- Walls, steps, seams or corners feel slippery or show visible growth.
- Free chlorine reads low, or disappears soon after you add it.
- Filter pressure climbs quickly and return flow weakens.
What usually causes it
Cause 01
Chlorine demand outran the dose
Warm water, rain, leaves, sunscreen and a crowd all consume chlorine. A single weekly tablet may not keep up during a hot, busy stretch.
Cause 02
Poor circulation left dead spots
Short pump runs, a clogged basket or returns aimed at one side leave corners and steps with little sanitized water moving past them.
Cause 03
Stabilizer is too high
CYA protects chlorine from sunlight, but at high levels it also slows the chlorine's ability to kill algae. Test CYA before simply adding more shock.
Cause 04
Organic load is feeding the bloom
Leaves, pollen, grass clippings and biofilm in the plumbing are food. Removing debris matters as much as the chlorine dose.
How to fix it, in order
Work through these steps once, with the pump running and the filter clean. Do not stack a second chemical dose before the first has circulated and you have retested.
- 01
Remove what you can see
Skim the surface, empty the baskets and brush walls, steps, seams and the waterline. Brushing breaks the algae's protective layer so chlorine can reach it.
- 02
Test before you shock
Record free chlorine, combined chlorine, pH, total alkalinity and CYA. Bring pH into the 7.2–7.6 range first; a shock dose works poorly at high pH.
- 03
Reach the right shock level
Use a chlorine/CYA chart or dose calculator based on your pool volume. The same shock number is not correct for every stabilizer level. Add it in the evening and keep the pump running.
- 04
Filter and clean around the clock
Run the pump 24/7 until the water clears. Clean or backwash the filter when pressure rises 8–10 psi above its clean baseline, not on a fixed weekly schedule.
- 05
Vacuum the dead algae
Once the bloom turns grey-blue and settles, vacuum slowly to waste if your filter allows it. Brush again and repeat the chlorine test; dead algae still creates demand.
- 06
Confirm with an overnight test
The pool is ready to reopen when the water is clear, combined chlorine is low and free chlorine holds overnight. If it keeps dropping, something is still consuming it.
What not to do
- Do not swim while the water is cloudy or you cannot see the main drain.
- Do not use copper algaecide as a substitute for chlorine; it can stain liners and steps.
- Do not backwash a sand filter every few hours just because the pool is green. Clean it when the pressure actually rises.
Keeping it from coming back
- Brush the walls and steps weekly, even when the water looks clear.
- Keep free chlorine inside the range for your CYA level and test after heavy rain or a crowd.
- Run the pump long enough to turn the pool over, and clean the baskets before flow drops.
- Shock after a party, a storm or any period when the pump was off.
When to stop and call a pro
Do not force it
Call a pool professional if the bloom returns within days, the water stays green after a correctly dosed shock, the pump is not moving water, or you see black or mustard-coloured patches that brushing does not disturb.
Compare local repair helpGeneral guidance for residential pools in Ontario. Chemical doses depend on pool volume and test results; follow the product label and keep gas, electrical and major shell work with licensed trades.