Spa water balance essentials: keep pH 7.2-7.6, alkalinity 80-120 ppm, sanitiser in range and water at 40C or below for safe, clear UK hot tub water.
Balanced spa water comes down to four numbers: pH between 7.2 and 7.6, total alkalinity of 80–120 ppm, a sanitiser residual of 3–5 mg/l free chlorine or 4–6 mg/l bromine, and calcium hardness around 150–250 ppm. Hold those four in range and the water stays clear, comfortable and safe – drift outside them and you get cloudiness, irritation, scale or corroded equipment. This guide explains what each parameter does, why the order you correct them matters, and how UK spa owners keep everything in range with a few minutes' work a week.
A hot tub holds only 800–2,000 litres and runs at up to 40°C. That small, hot volume means every bather adds proportionally far more oils, sweat and cosmetics than they would to a pool, and heat accelerates every chemical reaction. Where a pool drifts over weeks, a spa can swing out of balance overnight – which is why testing 2–3 times a week is the non-negotiable foundation of water balance.
Total alkalinity (TA) is the water's buffer against pH swings. With TA in the 80–120 ppm band, pH moves slowly and predictably; below it, pH bounces wildly after every dose and bathe; above it, pH locks stubbornly high. Always correct TA first – raise it with a total alkalinity increaser, and lower it with small, spaced doses of a pH & alkalinity reducer, retesting 20–30 minutes after each addition.
pH controls both bather comfort and how well your sanitiser works. Below 7.2, water stings eyes, irritates skin and slowly corrodes heaters and seals. Above 7.6, chlorine loses much of its killing power, scale forms and water turns cloudy. Nudge low pH up with pH plus and bring high pH down with pH minus – always in small doses with the pump running, because overshooting just means dosing back the other way.
Balance makes water comfortable; sanitiser makes it safe. Hold free chlorine at 3–5 mg/l or bromine at 4–6 mg/l at all times – a spa at 38°C with no sanitiser residual can grow harmful bacteria within hours. Add a weekly oxidising shock to burn off the dissolved bather waste that testing alone will not show. Balance and sanitisation work as a pair: perfectly balanced water with zero sanitiser is unsafe, and plenty of sanitiser at pH 8.0 is largely wasted.
Soft water (common in Scotland, Wales and the north-west of England) is aggressive and foams easily; raise it with a calcium hardness increaser at refill. Hard water (much of southern and eastern England) deposits scale on heaters and the waterline – keep pH tightly in range and dose a scale inhibitor at each fill. You typically only adjust hardness once per water change, not weekly.
pH 7.2–7.6, total alkalinity 80–120 ppm, calcium hardness roughly 150–250 ppm, and a sanitiser residual of 3–5 mg/l free chlorine or 4–6 mg/l bromine, with water at 40°C or below.
Alkalinity first, always. TA buffers pH, so until TA sits in the 80–120 ppm band, any pH adjustment will drift straight back out of range and waste chemicals.
Aeration from jets naturally drives pH upward, and high alkalinity accelerates it. Check TA – if it is above 120 ppm, bring it down and the pH creep will slow markedly.
Circulate for 20–30 minutes after dosing balancers, then retest. Once pH, alkalinity and sanitiser all read in range, the water is ready to use.