Quick answer
Test free chlorine, combined chlorine, pH, total alkalinity and CYA. Clean the baskets and filter, correct any imbalance, then run the pump continuously. Use a clarifier only after the filter is clean and the chemistry is right; some flocculants are not compatible with cartridge filters.
What you're seeing
- The water looks milky or hazy but is not necessarily green.
- You cannot clearly see the main drain or the bottom in the deep end.
- Filter pressure is above its clean baseline or return flow feels weak.
- Chlorine demand rises and the water does not sparkle after dosing.
What usually causes it
Cause 01
The filter is bypassing particles
A dirty cartridge, channeled sand or torn DE grid lets fine debris return to the pool. Pressure and flow tell you which part of the system to inspect.
Cause 02
pH or calcium is out of range
High pH and high calcium can create a milky haze that no amount of clarifier will remove until the water is balanced.
Cause 03
Circulation is too short
If the pump runs only a few hours a day, suspended particles never reach the filter often enough to be removed.
Cause 04
Dead algae or a recent treatment
A pool can look worse for a day after shock or algaecide because dead organic matter is being pushed through the filter. That is different from a new bloom.
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
Take a full set of readings
Record free chlorine, combined chlorine, pH, total alkalinity, CYA and calcium hardness. A single pH strip is not enough to explain persistent cloudiness.
- 02
Inspect the filter, not just the gauge
Empty the pump basket, then clean or backwash according to the filter type. Check for torn cartridges, channeled sand or a broken grid.
- 03
Balance before clarifying
Bring pH and alkalinity into range and restore free chlorine. If combined chlorine is high, shock and let the filter run.
- 04
Run the pump continuously
Give the filter 24–48 hours of continuous circulation. Watch the pressure; clean it again if it rises 8–10 psi above clean.
- 05
Clarify only when appropriate
If the filter is clean and chemistry is balanced, a clarifier can gather fine particles for the filter. Check the label for your filter type and never mix chemicals.
What not to do
- Do not add flocculant to a cartridge filter without checking the manufacturer's instructions; it can clog or damage the cartridge.
- Do not backwash a sand filter repeatedly without checking pressure, because you can reduce filtration efficiency.
- Do not swim when you cannot see the bottom or the main drain.
Keeping it from coming back
- Clean baskets before the pump struggles, not after flow stops.
- Backwash or clean the filter at 8–10 psi above its clean baseline.
- Keep pH, alkalinity and sanitizer in range and test after heavy use.
- Give the filter a deep clean at the start and end of the season.
When to stop and call a pro
Do not force it
Call a technician if the water stays cloudy after a clean filter and balanced chemistry, if sand or DE is returning to the pool, or if the filter tank, valve or multiport is leaking.
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.