Cloudy water is a symptom, not a diagnosis. It means something is suspended in your pool that shouldn't be — but that something might be algae too small to see, calcium that has fallen out of solution, dead organic matter your filter can't catch, or ordinary dust from last week's storm. Those four have different fixes, and two of the fixes make the others worse: shocking a pool that's cloudy from high pH drives the pH higher still, and pouring clarifier into a pool with no chlorine feeds the algae you haven't noticed yet.
So the useful question isn't "what clears cloudy water" — it's which of the four you have. That takes about ten minutes and a test kit, and it's the part most advice skips.
Which of the four is it? Start here
Test first — free chlorine, pH, alkalinity, calcium hardness and CYA — then read down this table and stop at the first row that matches. The order matters: it runs from the cause that's most common and most urgent to the one that's most harmless.
| If your test says… | The cause is probably | Confirm it by | Go to |
|---|---|---|---|
| FC below the minimum for your CYA (see the next section — this is not the same as "FC is above zero") | Early algae, or organic load your chlorine can't keep up with | An overnight chlorine loss test: FC at dusk, FC at dawn, pump running. Losing more than 1 ppm means something is eating it | Cause 1 |
| FC fine, but pH above 7.8 or calcium hardness above ~500 or you just used cal-hypo | Calcium precipitating out of solution | Run your numbers through the saturation index — above +0.6 and the water is shedding calcium | Cause 3 |
| Everything in range, and it got cloudy after rain, wind, heavy swimmer use or a nearby building site | Suspended debris too fine for your filter | It appeared within a day of the event and the water is grey-hazy rather than milky-white | Cause 4 |
| Everything in range, no obvious event, and it has been getting slowly worse for a week or more | The filter isn't keeping up | Check filter pressure against its clean baseline, and count your actual turnover hours | Cause 2 |
| Cloudy and tinged green, or the walls feel slippery | Algae proper, past the early stage | Brushing raises clouds from the walls that don't settle | The SLAM process |
More than one can be true at once — high pH and low chlorine travel together in a neglected pool. Work them in the order above: chlorine first, because it's the one that gets worse while you deliberate.
Fill a clear glass or white bucket from about a foot down and leave it somewhere still for a few hours. Fine grit settles into a visible layer — that's debris or dead algae, and it's a filtration job. The water stays uniformly milky — that's precipitate or live algae, both too fine and too buoyant to settle, and both chemical problems. It clears completely and looks fine — your water is clear and what you're seeing in the pool is an optical effect: sun angle, a dirty waterline, or a stained surface making clear water look grey.
Cause 1: the chlorine is lower than your test says
This is the most common cause and the most commonly missed, because the reading looks reassuring. "My free chlorine is 2 ppm, so chlorine isn't the problem" is only true if you know your CYA. Cyanuric acid — stabilizer — protects chlorine from sunlight by binding most of it into a reserve, which means the higher your CYA, the more total FC you need to keep the same small amount of active sanitizer in the water. At CYA 80, an FC of 2 ppm isn't a comfortable margin; it's a third of the minimum, and algae is already growing faster than your chlorine kills it.
| Your CYA | Minimum FC manual chlorine | Target FC | Minimum FC salt pool | Shock level if it's algae |
|---|---|---|---|---|
| 30 ppm | 2.5 ppm | 4 ppm | 2 ppm | 12 ppm |
| 50 ppm | 4 ppm | 6 ppm | 2.5 ppm | 20 ppm |
| 70 ppm | 5.5 ppm | 8 ppm | 3.5 ppm | 28 ppm |
| 80 ppm | 6 ppm | 9 ppm | 4 ppm | 32 ppm |
Computed from the same engine our calculators use — minimum is about 7.5% of CYA for manual chlorine, 5% for a salt pool, and shock level is 40%. Full reasoning in the chlorine / CYA relationship.
The fix. If FC is below the minimum for your CYA and the water is cloudy, treat it as early algae rather than hoping: raise FC to the shock level in that last column and hold it there, testing and topping up several times a day, until the water clears and you pass an overnight loss test. The shock calculator gives the dose for your volume, and the SLAM guide covers the full process — it's the same process whether the water is cloudy or properly green, just shorter.
If your CYA is above about 80, note that the shock level becomes both expensive and hard to hold. That's the point where draining and replacing part of the water is the cheaper route — the dilution calculator works out how much.
Cause 2: the filter isn't catching it
A filter can only remove what it can catch and only from water that reaches it. Three separate things go wrong here, and they need different fixes:
- Not enough hours. The water has to pass through the filter to be filtered — one full turnover a day is the usual target, and clearing a cloudy pool wants considerably more, up to running continuously. The run time calculator gives your real number from your volume and flow rate, which is a better answer than the "8 to 12 hours" everyone repeats.
- A filter that needs service. Compare your pressure gauge to the reading when the filter was last clean. Up by 8–10 PSI means clean or backwash it. Also worth knowing: a badly clogged filter doesn't just filter poorly, it cuts flow, so you get less turnover per hour as well as worse filtration.
- A filter that can't catch particles this fine. This is the one people don't expect, and it decides whether the pool clears in a day or never clears at all.
| Filter type | Catches down to | On fine cloudiness | What to do |
|---|---|---|---|
| Sand | ~20–40 microns | Struggles — the particles making your water cloudy are mostly smaller than this | Clarifier genuinely helps here; it clumps particles until they're big enough to catch. A cup of DE added through the skimmer sharpens a sand filter considerably |
| Cartridge | ~10–20 microns | Reasonable, and better once slightly dirty | Run it; clean only when pressure demands. Rinse rather than replace unless the pleats are damaged |
| DE (diatomaceous earth) | ~3–5 microns | Excellent — clears most cloudiness unaided | Backwash and recharge with fresh DE, then just run it. Clarifier is rarely needed |
Approximate media ratings; for comparison, the particles that make water look cloudy are typically in the 0.5–5 micron range, which is why filter type matters so much more than most advice admits.
Cause 3: something fell out of solution
Water holds dissolved calcium up to a limit, and that limit falls as pH, alkalinity, hardness and temperature rise. Push past it and calcium carbonate comes out as a suspension of microscopic crystals — the classic "milky" or "white-cloudy" pool, as distinct from the grey haze of debris. Left alone it settles onto surfaces as scale, and by then it's a much bigger job.
The catch is that no single reading looks alarming. Each of these scenarios has numbers a pool store would call fine:
| Scenario | pH | TA | CH | Temp | Saturation index | Result |
|---|---|---|---|---|---|---|
| Balanced The control — nothing is coming out of solution | 7.5 | 80 | 350 | 82 °F | -0.06 | Balanced |
| pH drifted up Classic late-summer SWG pool | 8.0 | 90 | 350 | 82 °F | +0.49 | Starting to precipitate |
| Hard fill water Topped up all season from a hard tap | 7.8 | 90 | 600 | 82 °F | +0.53 | Starting to precipitate |
| After cal-hypo shock Cal-hypo added calcium and pushed pH up | 7.9 | 90 | 500 | 86 °F | +0.59 | Starting to precipitate |
| Both, on a hot day Cloudy within hours, and it will scale | 8.0 | 100 | 600 | 86 °F | +0.82 | Scaling — cloudy, then deposits |
Calcite saturation index computed by our engine at CYA 40 — the same math behind the LSI / CSI calculator, which uses carbonate alkalinity rather than raw TA. Above about +0.6 the water is actively shedding calcium; below −0.6 it's dissolving it out of your plaster and grout instead.
The fix is pH, almost always. Look at the table: pH is the term that moves the index furthest and it's also the cheapest and fastest to change. Bring pH down to 7.4–7.6 with muriatic acid — the pH calculator gives the dose — and cloudiness from precipitation usually starts clearing within a day as the crystals redissolve. If calcium hardness is genuinely high (above ~600, or you're on hard fill water), you'll be fighting it all season, and partial replacement is the durable fix.
Usually the shock itself, not a coincidence. Cal-hypo adds about 0.7 ppm of calcium hardness for every 1 ppm of chlorine, and it's alkaline — so a heavy dose raises hardness and pH together, which is precisely the combination in the table above. Any shock also kills organic matter that was previously invisible, and dead cells are cloudier than live ones; that kind clears through the filter over a day or two. The tell is which: milky-white and it doesn't settle in a bucket means calcium — check pH first. Grey and it settles means dead organics — keep the filter running. If you shock often, switching to liquid chlorine avoids the calcium problem entirely, which is one of the arguments in liquid chlorine vs tablets.
Cause 4: it really is just particles
Sometimes the chemistry is genuinely fine and the pool has simply taken on more fine material than the filter can clear quickly: pollen in spring, dust and ash on the wind, runoff after heavy rain, sunscreen and body oils after a busy weekend, or grit from building work nearby. This is the only one of the four where reaching for a bottle is the right first move rather than a way of avoiding the diagnosis.
Even then, do the free things first: empty the skimmer and pump baskets, brush the walls and floor toward the main drain, backwash or clean the filter so it starts with capacity, and run the pump continuously. Most post-storm cloudiness clears on filtration alone within a day or two — faster on DE, slowest on sand.
Clarifier, flocculant, or neither
These two get recommended interchangeably and they work in opposite directions. Neither is a sanitizer, and neither fixes a cause — they're tools for getting particles out of water that is otherwise healthy.
| Clarifier | Flocculant | |
|---|---|---|
| What it does | Makes fine particles clump until the filter can catch them | Makes everything drop to the floor as a sludge |
| Then you | Run the filter — that's it | Vacuum to waste, with the filter bypassed |
| Time to clear | 1–3 days | Overnight to settle, plus the vacuuming |
| Water lost | None | Meaningful — you're vacuuming water out, and refilling changes your chemistry |
| Needs | Any filter; most useful on sand | A multiport "waste" setting, or a way to bypass the filter. Cartridge filters generally can't |
| Best for | Mild haze, balanced water, no rush | Heavy cloudiness where you want the pool usable this weekend |
| Don't use when | Chlorine is below minimum, or the cause is precipitation — fix the cause or you'll be doing this again next week | |
The honest position: clarifier is sold for a problem that is usually a chlorine problem or a filter problem. On a sand filter with genuinely balanced water and post-storm haze, it earns its money. Poured into a pool with FC below minimum, it's a few dollars spent making the water look temporarily better while algae keeps growing. If you don't know which of the four causes you have, a bottle of clarifier is the most expensive way to not find out.
How long should it take to clear?
Long enough that people give up too early and start adding things. Rough expectations, assuming the cause has actually been fixed and the pump is running continuously:
- Precipitation, after correcting pH: hours to a day. This one is often dramatic — you correct the pH in the evening and it's clear by morning.
- Dead organics after shocking: one to three days on cartridge or DE, longer on sand.
- Early algae being shocked: two to five days, and the water usually gets cloudier before it gets clearer as the algae dies. Judge progress by the overnight chlorine loss test, not by looking.
- Post-storm debris: a day or two on DE, two to four on sand — add clarifier if sand is the constraint.
If a pool hasn't visibly improved in 48 hours of continuous filtration with the cause corrected, something in the diagnosis is wrong. The usual culprits: the chlorine isn't actually holding (re-test at dusk and dawn), the filter is channelling or the media is worn out, or there are two causes and you've fixed one.
Not going cloudy again
Nearly all repeat cloudiness traces back to one of two slow drifts. CYA creeping up — from trichlor tablets or dichlor, each of which adds stabilizer every time you use it — quietly raises the FC you need while you keep dosing the same amount, until the pool is chronically under-chlorinated at a reading that looks unchanged. Test CYA a few times a season, not once. pH creeping up — the natural direction for almost every pool, and faster with a salt cell, an aerating feature, or fresh plaster — walks you toward the precipitation line. A little acid often beats a lot occasionally.
Beyond that it's unglamorous: keep FC in the range for your CYA rather than above zero, run the pump enough hours to turn the water over, clean the filter on pressure rather than on a calendar, and test after heavy rain or a party rather than waiting for the water to tell you. The test dashboard takes all your readings at once and gives every dose in one place, which makes the weekly version of this about five minutes.