Why is my pool cloudy?
Cloudy water is a symptom, and five things cause nearly all of it.
Cloudy water means something very small is suspended in it — early algae, dead algae, minerals dropping out of solution, or dirt the filter is not catching. Nearly all of it comes down to the five causes below, listed in the order they usually turn out to be true, and four of the five are fixed by chemistry and circulation rather than by anything you need to buy.
Start with a water test, not a product
Cloudy water looks the same whatever is causing it, so guessing is expensive. Test the water and let the app read the numbers, because the same reading that names the cause is the one that sizes the fix. A product bought before the test is a product bought for a cause you have not identified yet.
The five causes, in the order they usually turn out to be true
- There is not enough free chlorine for your stabilizer level. This is the most common cause by a wide margin, and it is early algae you cannot see yet. Stabilizer decides how much chlorine your pool needs, so a level that was plenty last month can be short now; the app sets your target against your stabilizer reading and sizes the dose.
- The filter is not keeping up. Too few pump hours, a dirty cartridge, a filter due for a backwash, or air getting into the system will all leave the water hazy while every chemical reading looks fine. Run the filter continuously until the water clears, and clean or backwash it once the pressure gauge sits well above its clean reading.
- Calcium is coming out of solution. When hardness and pH are both on the high side the water can no longer hold its calcium dissolved, and it appears as a fine white cloud, often a day or two after adding cal-hypo or soda ash. Bringing pH toward the low end of its range is the move, and the app tells you when your reading is in that territory.
- Dead algae is still in suspension after a shock that worked. The algae is gone, but what is left is too fine for the filter to catch on the first pass. Keep the filter running and give it time. This is the one case where a clarifier genuinely helps.
- Stabilizer is high enough that no chlorine level is doing much work. Past a certain point most of the chlorine you carry is locked up, and no amount you can safely add gets ahead of it. The answer is replacing some of the water, which the app sizes for you in gallons.
When a clarifier helps
A clarifier clumps particles too fine for your filter into pieces big enough for it to catch. That makes it useful for exactly one of the five situations above — the last of the haze once the cause is already fixed, usually dead algae or fine dust — and no use for the other four. It needs a clean filter running long hours to do anything at all, and it takes a day or two to show a result. It speeds up the end of the job; it never does the job.
Why a flocculant is a last resort
A flocculant is not a stronger clarifier, it is a different tool. It binds everything into a blanket that sinks to the floor, and that blanket has to be vacuumed out to waste — straight to the drain rather than through the filter — so you need a multiport valve and you will lose several inches of water doing it. On a cartridge filter there is no way to vacuum to waste at all, and pushing the sludge through the filter instead packs it solid and leaves the pool worse than it started. It also pulls pH down hard. It earns its place on a heavy debris load when you are set up for it, and almost nowhere else.
The short versionFour of the five causes of cloudy water are fixed by chemistry and circulation, which is why a bottle so rarely settles it. Test the water, keep the filter running, and let the app tell you which of the five you are looking at.
How to clear a green pool
A green pool is cleared by a level you hold, not by a dose you add.
A green pool has an established algae bloom, and it is the one situation the normal weekly routine cannot reach. Clearing it takes a few days and it works reliably when you do all four parts — a raised chlorine level held continuously, brushing, filtration, and one test at the end that tells you it is over. Nothing about a green pool means you did something wrong; a warm week and a few days of low chlorine is all it takes.
Why one dose cannot do it
Algae consumes chlorine. Pour in enough to reach a high reading and a bloom will eat through it within hours, so the water is green again in the morning and the number is back where it started. That is not a failed dose, it is the bloom feeding. Clearing a pool means holding the chlorine at a raised level continuously until there is nothing left to consume it, which is a different job from correcting a reading.
What a sustained hold means in practice
You raise the chlorine to a specific level, then test and top it back up to that same level several times a day, for as many days as it takes. The level is not a fixed number — it is set by your stabilizer reading, and the app works it out and sizes each top-up. What matters is that the water is never left to fall back down between doses, because every hour it spends low gives the bloom time to recover.
Brush every surface, every day
Algae grips walls, steps and the shaded corners behind ladders as a film that chlorine struggles to penetrate. Brushing breaks the film and puts the algae into the water where the chlorine can reach it, and it lifts settled debris for the filter. Brush the whole pool once a day while you are clearing, and give the steps, the corners and anywhere the sun does not reach the most attention.
Run the filter around the clock
Chlorine kills the algae and the filter removes it, so both have to be working. Run the pump continuously for the whole process and check the pressure daily, because a filter working through a bloom loads up fast and a clogged filter stops the water clearing no matter how good the chemistry is. Clean or backwash it whenever the pressure climbs well above its clean reading.
The overnight test is the finish line
Clear water is not the same as finished water. The test that tells you the bloom is dead is a comparison: record the chlorine at dusk, keep the pump running overnight, and read it again before the sun comes up. If the level has barely moved, nothing living is left in the water eating chlorine, and you are done. If it dropped, the bloom is still alive and you hold the level for another day. Running it overnight takes the sun out of the equation, so anything that consumed the chlorine was something alive.
Why algaecide is not the fix
Algaecide is sold as the green-pool product and it is the one thing on that shelf that will not clear a green pool. It is built to slow algae down in water that is already balanced, not to remove an established bloom, and it does nothing about the organic load a bloom leaves behind. The copper-based kinds can stain a plaster surface permanently, and the rest tend to foam. Held chlorine is what clears the water, and a clarifier can help the filter catch the dead algae once the chlorine has finished.
The short versionHold the level, brush, filter, and let the overnight comparison tell you when it is over. It takes days rather than one trip to the store, and it works.
Clarifier vs flocculant
Two products with similar names that do opposite things to your filter.
These two sit beside each other on the shelf, sound alike, and are not interchangeable. One works through your filter and one works around it, and reaching for the second when you meant the first is one of the easiest ways to make a cloudy pool worse.
What a clarifier does
A clarifier is a polymer, often chitosan, that makes fine particles stick together until the clumps are big enough for your filter to catch. Everything it gathers leaves through the filter, so the filter has to be clean and running long hours for it to do anything, and it needs a day or two before there is anything to judge.
What a flocculant does
A flocculant, usually aluminium sulfate, does close to the opposite. It binds everything into a heavy blanket that settles on the floor overnight, and that blanket has to be vacuumed out to waste — sent to the drain rather than through the filter — so you need a multiport valve and you will lose several inches of water. On a cartridge filter there is no vacuum-to-waste setting, which rules it out for that pool. It also pulls pH down noticeably, so plan on correcting that afterwards.
Which one, and when
- Reach for a clarifier once the cause is already fixed — the last of the dead algae after a clearing, or fine dust — and only with a clean filter running long hours. It speeds up the end of the job.
- Consider a flocculant only for a heavy debris load, only on a pool with a multiport valve, and only when you are ready to vacuum to waste and refill what you lose.
- Use neither while the cause is still live. If chlorine is low, the filter is short of hours, or the water is green, both products are being asked to do a job only chemistry and circulation can do.
Neither one sanitizes
It is worth saying plainly: both of these move particles around, and neither kills anything. Water that is clear is not the same as water that is safe to swim in. Your chlorine level is what makes it safe, and that is the number to settle first every time.
The short versionA clarifier helps your filter finish. A flocculant replaces your filter and costs you water. Neither substitutes for chlorine, and poolcntrl will never ask you to add either one.
What the pool store will tell you, and what your water actually needs
Six forces shape that printout, and only one of them is margin.
Take a sample to a pool store and you come home with a printed list and a basket to match it. It is worth understanding why the list looks the way it does, because most of what shapes it is structural rather than cynical — and because knowing the forces makes it much easier to separate the good advice from the padding.
Six things shape the printout
- The business is chemicals. Testing your water for free is how the store meets you, and the software behind the counter is built to turn a sample into a purchasable basket. A recommendation with nothing to sell attached to it cannot be printed.
- There is no product for the right answer. Replacing some of your water, running the pump longer, stopping tabs and brushing the steps are four of the most valuable things you can do for a pool, and no retailer can sell you any of them.
- Their measurement carries real slop, so the advice is padded. Counter tests for stabilizer run wide and strips run wider. Padding a dose is a reasonable answer to an uncertain reading — it just compounds on a chemical that cannot be taken back out.
- Complaints only come from one direction. Nobody returns to complain that their clear pool used a little more chlorine than it needed, and a green pool brings someone back upset. Every incentive points toward more chlorine, more often, plus something as insurance.
- Brand programs settle part of the answer in advance. Many stores run a seasonal step-by-step program tied to one product line, so part of the basket is decided before your sample is poured.
- It is built for a customer who visits monthly. Tabs, a weekly shock and a clarifier are a defensible plan for someone who will not test twice a week. It is worse chemistry and better compliance, and compliance is a real thing rather than an excuse.
The four pieces of advice no store can sell you
- Replace some of the water. It is the only way to bring stabilizer down, and it is the answer when a season of tabs has built up more than any chlorine level can work through. The app sizes it for you in gallons.
- Run the pump longer. A large share of cloudy water, and a good share of algae, is a circulation problem wearing a chemistry costume. Hours through the filter cost electricity rather than a product.
- Stop using tabs for a while. Every tab adds stabilizer, and stabilizer does not leave on its own. Tabs are convenient and they are fine in moderation, but once your stabilizer is at the top of its range they are the thing making the problem worse.
- Brush. Algae starts where the water moves least, and a brush costs nothing per use. A few minutes on the steps and the shaded corners prevents more trouble than most of what goes in the basket.
Where the store is genuinely better
One thing the counter does well is hand you a complete plan. Faced with a green pool it gives you a staged sequence and a day to come back, and that is worth a great deal when you are standing over water you do not understand. Crude chemistry with a plan beats correct chemistry with no next step, and any tool that wants to replace the trip has to offer the whole sequence rather than a single number.
What we do differently
poolcntrl answers the same question with no basket at the end of it. The engine can tell you to replace some water, wait two days, do nothing today, or stop using tabs, because none of those answers is worth more to it than any other. Your chlorine target comes from your own stabilizer reading rather than from a seasonal program, every amount is sized to your pool, and where the honest answer is that no product will help, that is the answer you get.
The short versionA pool store is not lying to you; it is answering a slightly different question, with a basket at the end. Judge any recommendation by whether it names a cause, and look twice at a list where every line has a price.
Every one of these articles hands the arithmetic back to the engine on purpose. When you want the actual amount for your own pool, put your readings through the free calculator.
