Hot tub water troubleshooting for UK owners: fix cloudy, foamy or green water fast with chlorine 3-5 mg/l, bromine 4-6 mg/l, pH 7.2-7.6 and TA 80-120 ppm.
Nine times out of ten, problem hot tub water comes down to one of four causes: low sanitiser, drifted pH, a dirty filter or tired, overloaded water. Test first, fix the chemistry, clean the filter, and only then reach for specialist treatments. Work through those steps in order and most cloudy, foamy or green water clears within 24 hours. This guide gives UK hot tub owners a systematic route from symptom to fix, with the exact target levels to aim for: free chlorine 3–5 mg/l or bromine 4–6 mg/l, pH 7.2–7.6 and total alkalinity 80–120 ppm.
Every troubleshooting session should begin the same way: dip a test strip. Adding chemicals to water you have not tested is how small problems become big ones. A set of 6-in-1 test strips reads sanitiser, pH, alkalinity and hardness in under a minute. Compare your readings against the targets above; whichever value is furthest out of range is usually your culprit. Once you have a reading, check it against the water chemistry reference and use the dosing calculator to correct anything out of range.
Correct total alkalinity first (80–120 ppm), then pH (7.2–7.6), then sanitiser. Alkalinity anchors pH, and pH controls how effective your sanitiser is – at pH 8.0 chlorine works at a fraction of its strength at pH 7.4, so water can look bad even when a strip shows plenty of chlorine present.
Cloudiness is the most common complaint and almost always means fine particles the filter has not caught: body oils, cosmetics, sun cream and dead organic matter. The fix, in order:
For a deeper walkthrough of causes and fixes, see our guide to cloudy hot tub water causes and solutions.
Foam is a surfactant problem: detergent residues from swimwear, lotions, and a build-up of body oils lower the water's surface tension so air whips into stable bubbles. Rinse swimwear in plain water rather than washing it with detergent, shower before bathing, and keep sanitiser in range so oils are broken down. A squirt of Foam Away knocks foam down instantly while you fix the underlying cause – but if foam returns within a day or two, the water is telling you it is overloaded and due a change.
Green tints or slimy shell surfaces mean algae or bacterial biofilm, and both mean your sanitiser has been at or near zero for a period. Shock the water hard, hold free chlorine at the top of the 3–5 mg/l range (or bromine at 4–6 mg/l) for 24 hours with the pump running, and clean the filter afterwards. If the green returns quickly, biofilm is hiding in the pipework: purge it with hot tub flush before draining and refilling.
Crusty deposits on the waterline and heater, or white flakes circulating in the water, point to hard water and high pH – common across much of England. Keep pH at 7.2–7.6, and dose a scale inhibitor at each refill to hold dissolved minerals in suspension. Scale on the heater element is more than cosmetic: it insulates the element, raises running costs and shortens its life.
Chemicals cannot rescue exhausted water. Total dissolved solids climb with every bathe, and once the water is saturated, problems recur no matter how carefully you dose. As a rule of thumb, UK hot tubs need a full drain and refill every 3–4 months – sooner with heavy use. If you are troubleshooting the same symptom for the third time in a fortnight, stop treating and start draining. Run the water no hotter than 40°C; hotter water consumes sanitiser faster and makes every problem here worse.
Usually because pH has drifted above 7.6, which cripples chlorine's killing power, or because the filter is greasy and recirculating fine particles. Rebalance pH to 7.2–7.6, shock the water and deep-clean the filter.
With correct chemistry, a shock dose and a clean filter, most cloudy or foamy water clears within 24 hours of continuous filtration. Green water can take 24–48 hours. If nothing improves after 48 hours, drain and refill.
Not recommended. Cloudy water usually means sanitiser is depleted or overwhelmed, so bacteria may not be controlled. Wait until the water is clear and a test confirms sanitiser and pH are back in range.
Test 2–3 times a week as a minimum, plus before bathing after any gap of a few days. Regular testing catches drift early, when a small top-up dose fixes it – rather than a full troubleshooting session.
This guide is written and maintained under the ClearSpa editorial policy. Dose rates and target ranges mirror the product labels current at the time of the last review.