The 5 most common hot tub water problems - cloudy, foamy, green or smelly water and low sanitiser - with fixes and UK targets: chlorine 3-5 mg/l, pH 7.2-7.6.
The five hot tub water problems UK owners hit most often are cloudy water, foam, green water, bad smells and a sanitiser level that will not hold. Almost all of them trace back to the same root causes – unbalanced water, a dirty filter or biofilm in the pipework – and the same targets fix them: free chlorine 3–5 mg/l or bromine 4–6 mg/l, pH 7.2–7.6 and total alkalinity 80–120 ppm. Here is how to diagnose and clear each one.
Cloudiness means fine particles the filter is not catching: dead skin, cosmetics and dust suspended in unbalanced water. First test and correct – alkalinity to 80–120 ppm, then pH to 7.2–7.6, then sanitiser to range. Rinse or deep-clean the cartridge with a filter cartridge cleaner, shock the water, and let a clarifier coagulate what remains so the filter can grab it. If cloudiness keeps returning days after every refill, suspect biofilm – see the fix under problem 5. Full walkthrough: cloudy hot tub water causes and solutions.
Foam is surfactants – detergent residue from swimwear, cosmetics, body oils – whipped up by the jets, and it gets worse as total dissolved solids build in ageing water. Shock the tub, knock the foam down with anti-foam for an instant fix, and rinse swimwear in plain water instead of washing it in detergent. Persistent foam in water more than three months old means it is time to drain and refill. More detail: fixing foamy hot tub water fast.
Green water is almost always algae, and algae only gets a foothold when the sanitiser has been low for days. Test first: if chlorine is near zero, correct pH, then shock hard and run the pumps continuously, cleaning the filter halfway through. A green tinge with metallic staining after a fresh fill can instead be dissolved metals from the supply – a scale and stain inhibitor deals with that. The step-by-step recovery is in fixing cloudy and green hot tub water.
A harsh "chemical" smell is not too much chlorine – it is chloramines, the spent by-product formed when chlorine binds to sweat and oils. A musty or sour smell is bacteria, usually in the pipework. Either way the fix starts with a shock: non-chlorine shock weekly keeps chloramines away, while a musty smell that returns points to biofilm (problem 5). If the smell survives shocking, drain, flush and refill.
If you dose chlorine or bromine and it vanishes by the next day, something in the water is consuming it. Fresh causes are heavy bathing loads and old, saturated water. The chronic cause is biofilm – the bacterial slime inside pipework where most hot tub bacteria actually live, sheltered from sanitiser. Before your next water change, run a hot tub flush through the pipework, then drain, rinse and refill; the biofilm flush guide covers dosing and timings. On a fresh fill with clean pipes, hold chlorine at 3–5 mg/l or bromine at 4–6 mg/l and the level will finally stick.
Work through faults methodically with the hot tub water troubleshooting guide, and keep the routine honest with the hot tub maintenance checklist.
Persistent cloudiness after refills usually means biofilm in the pipework re-seeding the fresh water. Flush the pipework before the next drain-down; day-to-day cloudiness is normally unbalanced water or a dirty filter.
No. Green water means algae or metals and, if sanitiser is low, potentially harmful bacteria. Shock, filter and rebalance until the water is clear and chlorine reads 3–5 mg/l (or bromine 4–6 mg/l) before bathing again.
Chloramines – used-up chlorine bound to bather waste – not an overdose. Shock the water to oxidise them; the smell and eye stinging disappear once combined chlorine is removed.
Roughly every three months for a typical family tub, sooner with heavy use. Always run a pipework flush just before draining so the fresh fill is not contaminated by old biofilm.