Why Your Local Water Hardness Changes Everything
Most pool chemical calculators ignore one critical variable: your starting water. When you top up your pool from the tap, you're adding the mineral profile of your local supply — including calcium, magnesium, and alkalinity. A pool in London (tap hardness ~360 mg/L) needs a fundamentally different treatment plan than one in Manchester (~68 mg/L), even if both pools show identical test strip results today.
Ignoring your source water leads to calcium scaling on pool surfaces (white crust on tiles and fittings) in hard water areas, or to aggressive corrosion in soft water areas where under-mineralised water strips minerals from your pool walls, pump, and heat exchanger.
Pool Water Chemistry: The 5 Parameters
pH (7.2–7.6)
The most critical parameter. pH affects every other chemical's effectiveness. Low pH causes eye irritation and equipment corrosion; high pH makes chlorine ineffective.
Free Chlorine (1–3 ppm)
Your primary sanitiser. Must be maintained daily. At <0.5 ppm, algae blooms within 24–48 hours. Stabilised chlorine (with cyanuric acid) lasts longer outdoors.
Total Alkalinity (80–120 mg/L)
Alkalinity "buffers" pH and prevents it from swinging. Adjust alkalinity before pH — they're tightly coupled.
Calcium Hardness (200–400 mg/L)
Too low: water is "hungry" and dissolves minerals from plaster, grout, and metal. Too high: white calcium deposits on all pool surfaces.
Cyanuric Acid (30–50 ppm)
Protects chlorine from UV degradation in outdoor pools. Without it, sunlight can destroy 95% of chlorine within 2 hours. Indoor pools don't need it.
Hard Water vs Soft Water Pools: Key Differences
| Issue | Hard Water (>200 mg/L) | Soft Water (<100 mg/L) |
|---|---|---|
| Calcium scaling | High risk — white deposits on tiles, ladders, waterline | Low risk |
| Corrosion | Low risk | High risk — attacks plaster, metal, and heat exchangers |
| pH stability | Moderate — high alkalinity buffers pH well | Poor — pH swings rapidly |
| Chlorine use | Normal | Higher chlorine demand due to pH instability |
| Filter cleaning | More frequent (calcium clogs filter media) | Less frequent |
How Often Should You Test Pool Water?
In the UK and Northern Europe, where pools are primarily outdoor and seasonal (May–September), test pH and chlorine at least 3×/week during swimming season. Test alkalinity, calcium hardness, and cyanuric acid monthly. After heavy rain, refilling, or heavy bather load, test all parameters before the next swim.
In hard water areas (London, Essex, Kent, East Anglia), expect to fight calcium scaling throughout the season. A sequestering agent added at the start of season can help keep calcium in solution rather than depositing on surfaces.