London, Kent, Essex, Hertfordshire: hard water makes calcium and scale your daily concerns. Here is the balanced approach for the long term.
South East England runs from very hard to extremely hard for tap water. Calcium hardness in the supply often exceeds 300 ppm, sometimes 400. Filling a hot tub from the mains in those postcodes means you start above the recommended ceiling, before you have added a single chemical. Here is the strategy that works.
Test the fill water before it goes in. If calcium hardness is above 300 ppm, you have hard-water problems waiting to happen.
The symptoms: scale on the heater element, milky cloudiness when pH drifts up, rough deposits on the waterline, and pump impeller wear over time.
Ignoring it does not make it go away. Hot water concentrates the issue compared with cold mains because evaporation leaves the calcium behind in the tub.
Use a hose-end pre-filter. The carbon-and-resin canister type catches a chunk of the calcium and brings the start point down to a manageable level. Costs around 20 pounds and lasts a few refills.
Dose the new water with a sequestering agent (a stain and scale inhibitor based on phosphonates). It binds calcium loosely so it stays in solution rather than depositing on heaters. Top up monthly.
Keep pH on the lower end of band (7.2 to 7.4). The slightly more acidic water suppresses scale formation. The trade-off is slightly faster sanitiser consumption, which is fine.
Wipe the waterline with a soft cloth weekly. The scale deposits there first because the air-water interface evaporates fastest.
Descale the heater element annually. Drain the tub, fill with a 5% citric acid solution, run for an hour, drain, refill. The element comes out looking new.
Replace the cartridge filter slightly more often (every 12 to 15 months instead of 18). Hard water clogs filter pleats with mineral deposits in addition to organic build-up.
Yes, but only the sodium-exchange type. The water comes out at almost zero hardness, which is then too low. You have to add calcium back in measured doses. Most owners find the hose-end pre-filter is enough.
No. Both sanitisers behave the same in hard water. The deposits on heaters and waterlines are the same regardless of which sanitiser you use.