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If your hot tub water has turned cloudy, milky or foamy, in the vast majority of cases there is a single cause: the pH has drifted outside the 7.0 - 7.6 range and the sanitiser has stopped working. It is not visible dirt, it is chemistry that has stalled. The good news is that you can measure it in two minutes with a test kit, and fix it in most cases without draining the tub.
Here is what is actually happening in the water, how to tell which of the four cases you are dealing with, and when you should stop treating the water and simply change it. Because sometimes that is the right answer, and nobody tells you.
Why hot tub water turns cloudy: pH comes first
A hot tub is not a small swimming pool. The conditions are far more demanding, and our technical support sheet sums them up in three numbers: the water sits between 36 and 40 °C, the volume is under 2,000 litres, and the organic load per litre is much higher, because half a cubic metre of water holds three or four people.
Heat accelerates everything. At 37 °C bacteria multiply at a rate they never reach at 25 °C, and the sanitiser is consumed far more quickly. That is why an outdoor pool holds up for days while a covered hot tub, once the treatment stops, changes appearance within hours.
pH is the parameter everything else depends on, and it is the part almost everyone skips. The bromine or chlorine you add only works inside a narrow window: above 7.6 the sanitiser loses most of its effectiveness even when the amount in the water is correct. You keep adding product, the test says it is there, and the water stays cloudy. The product is not faulty, the water is out of range.
The value to hold is 7.0 - 7.6, and it comes from our technical support sheet on water treatment. The World Health Organization guidelines for safe recreational water environments, volume 2, devote a separate chapter to heated tubs precisely because temperature and small volume make them a different case from swimming pools.
The four cases, and how to tell them apart
Look at the water and find your case. The difference between white cloudy and green cloudy is not cosmetic: they are two different problems with two different solutions.
Milky or whitish water: pH or filter
This is the most common case and almost always the least serious. The water is opaque but has no colour: it looks like diluted milk. The causes, in order of probability: pH out of range, a dirty filter, or hard water with limescale precipitating in suspension.
Start with the pH test. If it is above 7.6, correct it and wait for a full filtration cycle before judging the result. If the pH is fine, the filter is the culprit.
Green water: that is algae
Green means algae, which tells you the sanitiser has been absent long enough for them to take hold. It typically happens after a few days with the tub sitting unused, or after leaving it uncovered in the sun.
Correcting the pH is not enough here: you need an algaecide. Our catalogue product for this is SimplyBlu, € 68, which combines an algaecide action with limescale and organic matter control. The dosage from the support sheet is 10 ml per 1,000 litres, weekly, poured in with the filtration running so it distributes.
Clear water with foam on the surface
This case deserves a clarification, because foam is not dirt. It is surfactant: traces of shampoo, shower gel, body lotion and fabric softener from swimwear, carried into the tub by the bathers and whipped up by the jets.
People who write to me about foam often do not even see it as a problem, and it is. The marks it leaves along the walls are hard to remove, and the oily film that forms ends up in the filtration circuit. The skimmer fitted to our tubs limits the damage, because it draws surface water towards the filters, but it does not eliminate it.
The practical rule is the dullest one there is, and it works: shower before getting in, and rinse swimwear without fabric softener. Surfactants are what create foam, and you are the one bringing them in.
One prohibition is worth repeating because it is expensive to ignore: ordinary essential oils must never go into the tub. They clog the filters, leave oily films and damage the surfaces. Fragrances made for hot tubs exist and are formulated specifically not to foam.
Clear water with an unpleasant smell
This is the case that no product poured into the tub will solve, because the problem is not in the water: it is in the pipework. Bacterial biofilm forms inside the hydraulic circuit, where the sanitiser barely reaches, and keeps releasing back into the water everything you keep treating.
What you need is a circuit sanitiser, not a water disinfectant: in our catalogue that is the BL-SPASAN spa sanitiser, € 54. The support sheet recommends it for smells that persist despite correct treatment, and when reopening after a long shutdown.
The filter: the cause nobody suspects
If the pH is correct and the water stays hazy, in most cases the filter has stopped filtering. It is not a fault, it is skipped maintenance.
The support sheet calls for cleaning every 30 days and gives a life of one to two years for a well kept cartridge. The procedure is simple: pull the filter out of the skimmer, rinse it with a medium pressure jet aimed downwards between the pleats, and let it dry completely before putting it back.
That detail about aiming the jet downwards is not pedantry: the other way round you push the dirt deeper into the pleats instead of flushing it out.
A clogged filter does more than spoil the water. It cuts the flow rate, makes the massage feel weak and forces the pumps to work harder. A torn filter is worse still, because it lets debris into the circuit where it reaches impellers and bearings: that one gets replaced immediately, not washed.
Water hardness, if the problem keeps coming back
If you correct everything and the water turns hazy again within days, look at the hardness of your mains water. Hardness is measured in French degrees, where 1 °fH equals 10 mg/l of calcium carbonate, that is 10 ppm. In countries using other scales, 1 °fH is 0.56 °dH and roughly 0.7 grains per gallon.
The threshold that matters is in the support sheet: up to 8 °fH nothing is needed, between 8 and 12 a softener is advisable, above 12 °fH it becomes necessary. Past that value limescale precipitates, clouds the water and settles on heating elements and pipework, and no corrector poured into the tub makes up for water that is too hard to begin with. Your water utility publishes the figure, usually in the water quality report for your area.
When to stop treating and change the water
This is the part that chemical suppliers are not eager to write, and it is the most useful advice I can give you.
The support sheet is explicit: for tubs under 2 cubic metres, a complete water change every 4 - 6 weeks costs less than keeping the water alive with products beyond that period. This is not an environmental stance, it is arithmetic: past a certain concentration of dissolved organic matter, the amount of chemistry required grows faster than the cost of fresh water.
There is a second reason, less obvious. Mains water arrives with an already balanced pH, so a partial or complete change is also the simplest way to bring the pH back into range, often without using correctors at all. The support sheet says it plainly: with clean drinking water at each change, pH correctors become almost unnecessary.
We worked out what that costs in a separate guide: you will find the numbers in what it costs to fill a hot tub and change the water, tanker delivery included if you have no tap nearby.
One last point about the water you start with, because this is a mistake I see often: use clean drinking water from the mains. Wells and uncontrolled natural sources bring in sediment and bacteria, and you end up chasing with chemistry a problem you created when filling.
What to keep at home, and what not to
You do not need eight different products. Routine hot tub treatment rests on three things.
The recommended sanitiser is slow release bromine tablets of 20 grams each, the BL-SPABRO bromine tablets at € 79. Compared with chlorine they are colourless, odourless, leave no marks on surfaces or swimwear, and are more stable above 35 °C, which is exactly a hot tub's operating temperature. Chlorine above that threshold degrades faster and forces you to check the pH more often. The tablets go into the skimmer compartment, dissolve over the course of a week and are topped up on that schedule. You will find them all in the cleaning and maintenance category.
Then you need a test kit for the pH, which is the tool that makes guessing unnecessary, and the algaecide only when you actually need it.
For debris on the bottom, sand and insects in outdoor tubs, chemistry is not the answer: use a vacuum, the BL-SPAVAC spa vacuum at € 133.
Frequently asked questions
Why does hot tub water turn cloudy?
In most cases because the pH has drifted outside the 7.0 - 7.6 range and the sanitiser has stopped working, even though it is present in the water. The other two frequent causes are a dirty filter and mains water that is too hard, above 12 °fH.
How do I get my hot tub water clear again?
In this order: measure the pH and correct it, clean the filter, then wait for a full filtration cycle before judging. If it is still hazy after a complete cycle, the pH was not the problem and you should look at hardness or change the water.
Why is my hot tub water milky or whitish?
It is opaque but has no colour, so it is not algae: it is almost always pH out of range or a clogged filter. If it turns milky again every few days despite corrections, it is limescale in suspension and the real problem is the hardness of your mains water.
My hot tub water is green: what do I do?
That is algae, which means the sanitiser was missing long enough for them to take hold. You need an algaecide, not just a pH correction: SimplyBlu at 10 ml per 1,000 litres, with the filtration running so it distributes.
Why does my hot tub foam?
Because of surfactants carried in by the bathers: traces of shower gel, shampoo, body lotion and fabric softener from swimwear, whipped up by the jets. It is not dirt and it is not a fault of the tub. Showering before getting in and rinsing swimwear without fabric softener fixes it at the source.
Can I use bubble bath in a hot tub?
No. It foams uncontrollably, leaves marks along the walls and the film ends up in the circuit. Fragrances made for hot tubs exist and are formulated not to foam, whereas ordinary essential oils are banned outright because they clog filters and damage surfaces.
The water is cloudy but bromine reads correctly: why?
Because the sanitiser reading means nothing if the pH is out of range. Above 7.6 the bromine is present and inactive, so adding more will not fix it and only costs you money. Measure the pH before adding any product.
Can I use chlorine instead of bromine?
Technically yes, but our support sheet advises against it for two reasons: at hot tub temperatures it is less stable and forces more frequent pH checks, and indoors it leaves the typical swimming pool smell, which becomes unpleasant fast in a basement or a conservatory.
Quick summary
If the water is not what it should be, work through this order and stop as soon as it clears.
- Measure the pH. If it is outside 7.0 - 7.6, correct it or do a partial change with mains water, then wait for a filtration cycle.
- Check the filter. Clean every 30 days, jet aimed downwards between the pleats, dry completely. If it is torn, replace it.
- Identify the colour. White means pH or filter, green means algae and calls for an algaecide, foam means surfactants and calls for a shower before getting in.
- If the smell persists with clear water, the problem is biofilm in the pipework: you need a circuit sanitiser, not a water product.
- If the problem returns every few days, check the hardness of your mains water: above 12 °fH you need a softener upstream.
The next step, if the water is more than four to six weeks old, is to change it rather than keep treating it. The most common mistake is the opposite: persisting with products on exhausted water, spending more than a refill would cost.
If the water still refuses to clear after all this, the problem may be in the circuit or the filtration system, and that needs eyes on site: get in touch describing the colour, the values you measured and how long it has been going on. The people who answer have been installing and repairing these tubs for years, and you can read our story on about us.
The catalogue data in this guide (prices, dosages and formats) is read from the product pages and updates with the catalogue: what you see is the current value. It remains indicative because it refers to standard configurations. Chemical values and maintenance intervals are general guidance for domestic hot tubs under 2 cubic metres: dosages and procedures must always be checked against the product manual and your tub's manual. Professional and public installations are subject to different regulatory requirements, including sand filtration. Water hardness thresholds are given in French degrees, the scale used in Italy: check the equivalent scale used by your local water utility. Nothing reported here replaces an assessment by a qualified technician for your specific case.
