Cloudy pool water: what to check, in order
Cloudy water is a symptom, not a diagnosis. The mistake most people make is reaching for a clarifier first. Work through filtration and chemistry in order and you will usually find the cause in the first two steps.
Step 1: rule out filtration and circulation
Most cloudy pools are under-filtered, not under-dosed. Check these before touching chemistry:
- Filter pressure. If the gauge reads more than roughly 8–10 psi above its clean baseline, the filter is loaded and needs backwashing or cleaning.
- Runtime. A pump that runs too few hours never completes a full turnover. Calculate your turnover time.
- Suction and skimmer. A blocked skimmer basket or a pump drawing air will drop flow without the pressure gauge telling you.
- Dead spots. If returns are misdirected, some of the pool barely circulates.
Step 2: start from the right volume
Every chemistry decision scales with water volume, and a wrong volume quietly ruins every dose you add. If you have not measured it properly, do that first.
Get your pool volume in gallons →
Step 3: test, and write the numbers down
Test at the same time of day, ideally in the morning before the pool is used. You want free chlorine, pH, total alkalinity, calcium hardness and cyanuric acid (CYA). The pattern across those numbers matters more than any single reading.
| Reading | What a problem looks like | Where to go next |
|---|---|---|
| Free chlorine | Low relative to your CYA level — sunlight and bather load outrun the sanitiser | Chlorine calculator (pending review) |
| pH | Outside roughly 7.2–7.8 — affects how well chlorine works and how comfortable the water is | pH calculator (pending review) |
| Total alkalinity | Very low — pH becomes unstable and swings | Alkalinity calculator (pending review) |
| Calcium hardness | Very high — can cloud the water directly on top of anything else going on | See "high calcium" below |
| CYA | Very low (chlorine burns off fast) or very high (chlorine stops working efficiently) | CYA calculator |
Step 4: the common causes, ranked
- Insufficient filtration. Not enough runtime, a dirty filter, or poor circulation. Most common by a wide margin.
- Early algae. Before water turns green it goes dull and then cloudy. If the walls feel slippery when you brush, treat it as algae and read the algae identification methods.
- High calcium hardness. Especially after topping up with hard fill water. It shows up as a milky haze that does not clear with filtration alone.
- High pH with high alkalinity. Scale can precipitate and cloud the water.
- Dead algae or debris. Fine particles that a sand filter in particular struggles to catch — this is the one case where a clarifier genuinely helps.
Step 5: what not to do
- Don't add clarifier before checking the filter — you will mask the symptom and keep paying for chemicals.
- Don't shock while the filter is loaded; the debris stays in the water.
- Don't adjust pH and alkalinity and chlorine all at once. One change at a time, then retest.
- Don't mix products that could react. Never mix chlorine with acid, and always add chemicals to water rather than water to chemicals.
Why this page has no dose table
Water clarity crosses into water chemistry and, at the margin, into safety. We publish the diagnostic order immediately, and we publish dose amounts only after a Certified Pool Operator has reviewed them. See the review status of every formula.
Related
- Pool volume calculator
- How long to run a pool pump
- CYA calculator
- How to backwash a pool filter — the filter is step 1 for a reason
- Pool maintenance schedule
- Why is my pool cloudy? — match the look to the cause
- Black algae in pool
- Mustard algae in pool