The test that splits the problem
Test total and free chlorine and subtract. Combined chlorine above 0.4 ppm, the level at which CDC’s Model Aquatic Health Code (5th edition, 2024) says to act, means the water is loaded with spent chlorine compounds and the fix is chemical. Combined chlorine near zero, with free chlorine in band, means the chemistry is fine and the cloud is physical — filtration, flow, or a balance problem throwing precipitate.
That single subtraction saves most of the guesswork, and it is the reason a strip is not good enough here: a strip gives combined chlorine only as total minus free, read off colour blocks too coarse to tell 0.2 ppm from 0.4. An FAS-DPD drop test reads it down to 0.2 ppm.
When it is chemistry
High combined chlorine after a hot weekend of heavy bather load is the classic. Take it to breakpoint in one dose at dusk — the shock calculator works out how much — and leave the pump running overnight. The water usually clears by the following afternoon.
When it is filtration
Three failures look identical from the deck: a filter that is genuinely dirty, a filter that is being driven too fast to catch anything, and a pump that is not running long enough. The pressure gauge only tells you about the first. Work out the filtration rate — if it is over the filter’s design rate (about 20 gallons per minute per square foot on a sand filter, about 2 on DE and about 1 on a cartridge, on Hayward’s published ratings), fine particles are going straight through no matter how clean the filter is.
| Symptom | Cause | Fix |
|---|---|---|
| Pressure high | Dirty media | Backwash or clean |
| Pressure normal, still cloudy | Channelled sand or torn grid | Strip and inspect |
| Clears then returns daily | Run time too short | Two turnovers a day |
| Never clears at any run time | Filter over-driven | Slow the pump or add area |
When it is balance
High pH and high calcium hardness together throw a fine calcium precipitate that hazes the whole pool and feels slightly gritty on the plaster. Bring pH back into the 7.2 to 7.8 band first and much of it redissolves. A pool that clouds within hours of every cal-hypo shock is telling you the same thing from a different direction.
The order to work
- Test free and combined chlorine, and pH.
- Correct pH if it is out of band — do this before anything else.
- If combined chlorine is high, shock to breakpoint.
- Clean the filter and run it continuously for 24 hours.
- Only then consider a clarifier or flocculant.
Where clarifiers actually help
A clarifier clumps fine particles so the filter can catch them. It is genuinely useful on a pool that has just been shocked and has a working filter. It is useless — and an expensive way to be disappointed — on a pool with a channelled sand bed or a run time of three hours. Fix the filtration first; the clarifier is the last step, not the first.