Hot tub water changes naturally as heat, bathers and evaporation shift its chemistry. What actually drives the drift and the routine that keeps it stable.
Hot tub water changes naturally because it is a small, hot, hard-working body of water: every soak adds body oils and sweat, heat and jets drive off sanitiser and push pH upwards, and evaporation concentrates everything that is left behind. None of that is a fault – it is ordinary spa chemistry, and a simple routine keeps it in check: test two to three times a week, hold chlorine at 3–5 mg/l (or bromine at 4–6 mg/l), pH at 7.2–7.6 and total alkalinity at 80–120 ppm, shock weekly and refill roughly every three months.
Each bather rinses body oils, sweat, dead skin, deodorant and traces of detergent into a few hundred litres of hot water. That organic load is what your sanitiser spends most of its time oxidising – which is why a busy weekend can take a comfortable chlorine reading to zero by Monday. If your sanitiser keeps vanishing faster than expected, that appetite has a name: chlorine demand. A weekly dose of non-chlorine shock burns off the accumulated waste before it can cloud the water or tie up your sanitiser.
Water at 37–40°C behaves very differently from a swimming pool. Heat speeds up every chemical reaction, so sanitiser is consumed faster; the jets aerate the water, which strips out dissolved carbon dioxide and nudges pH steadily upwards; and warm water is exactly what bacteria prefer, so any lapse in sanitiser shows up quickly. This is why hot tub pH nearly always drifts up rather than down, and why holding total alkalinity at 80–120 ppm matters – it buffers those swings so pH moves slowly enough for you to correct it.
Evaporation removes pure water but leaves every dissolved mineral and chemical by-product in the tub, and each top-up adds more. The running total is measured as total dissolved solids (TDS), and as it climbs, water gradually becomes harder to balance: sanitiser works less efficiently, foam appears more easily and clarity dulls even when every reading looks right. That slow build-up is why no amount of dosing makes old water behave like fresh water – see TDS and when to drain for the detail.
Gradual change is normal; sudden change is information. Water that turns cloudy overnight, a sanitiser reading that collapses within hours of dosing, or a musty smell from the jets all point to a specific cause – typically a spent filter, unbalanced water or biofilm – rather than ordinary drift. Treat those as troubleshooting jobs, not reasons to keep adding chemicals.
Heat, jet aeration and evaporation work on the water constantly: sanitiser breaks down, aeration pushes pH upwards, and evaporation concentrates dissolved solids. An unused tub still needs testing two to three times a week and a steady sanitiser residual.
The jets aerate the water and strip out dissolved carbon dioxide, which naturally pushes pH upwards. Holding total alkalinity at 80-120 ppm buffers the drift so pH moves slowly enough to correct with small doses.
Roughly every three months for typical UK use, and sooner with heavy bather loads. Rising total dissolved solids eventually make old water hard to balance no matter how carefully you dose, so a fresh fill is the real fix.
Keep free chlorine at 3-5 mg/l or bromine at 4-6 mg/l, pH at 7.2-7.6 and total alkalinity at 80-120 ppm, testing two to three times a week and after every heavy session.