Spa chemicals explained: what sanitisers, shock, pH and alkalinity adjusters do, with UK targets - chlorine 3-5 mg/l, bromine 4-6 mg/l, pH 7.2-7.6.
Spa chemicals fall into four working groups: sanitisers that kill bacteria, oxidisers (shock) that burn off bather waste, balancers that keep pH and alkalinity in range, and specialist treatments for specific problems like foam or scale. Understand what each group does and dosing stops being guesswork. This guide explains every category UK spa owners will meet, with the target levels that matter: free chlorine 3–5 mg/l or bromine 4–6 mg/l, pH 7.2–7.6 and total alkalinity 80–120 ppm.
Sanitisers maintain a constant disinfectant residual that kills bacteria as it enters the water. In a spa running at up to 40°C – ideal bacterial growth temperature – this residual is what keeps the water safe between doses. UK spa owners choose between two GB BPR-approved systems:
Pick one system and stay with it until a water change – bromine and chlorine products must never be mixed in dry form.
Sweat, body oils, cosmetics and sun cream dissolve into spa water where the filter cannot remove them. Left alone, this organic load consumes your sanitiser, feeds cloudiness and creates the combined chlorine that causes the harsh \"chemical\" smell. A weekly dose of non-chlorine shock oxidises that waste away, works alongside both bromine and chlorine, and typically allows bathing again after 15–20 minutes of circulation. Shock weekly and after any heavy bathing session.
Balancers do not disinfect anything – they create the conditions where the sanitiser works and the water feels comfortable:
These fix specific problems rather than forming part of the weekly routine: anti-foam knocks down surfactant foam, clarifiers coagulate fine particles so the filter can catch them, scale inhibitors keep dissolved minerals in suspension in hard-water areas, and pipework flushes purge biofilm before a drain-down. Useful tools – but if you need them constantly, the underlying chemistry needs attention first.
None of the above can be dosed correctly without knowing your numbers. Dip 6-in-1 test strips 2–3 times a week, correct alkalinity first, then pH, then sanitiser, and log the readings. For the full picture of how the four parameters interact, see our spa water balance essentials guide.
Five essentials: a sanitiser (bromine or chlorine), a shock/oxidiser, pH plus and minus, an alkalinity increaser and test strips. Everything else is a situational extra.
Sanitiser maintains a constant residual that kills bacteria around the clock. Shock is a periodic oxidising dose that burns off the dissolved bather waste sanitiser alone cannot remove. A safe spa needs both.
Yes, used as directed. Store them cool, dry and separately, never mix products directly, add chemicals to water (never water to chemicals) and keep them away from children. Properly dosed water is far safer than untreated water at spa temperatures.
Use spa-specific products. Pool chemicals are formulated and dosed for vastly larger volumes, so overdosing a 1,000-litre spa is easy, and some pool sanitisers behave poorly at 38–40°C spa temperatures.