By The Zagosa Team. Published .
Reviewed or updated .
Soft mains water feels like a bonus. No furred kettle, no chalky marks on the shower screen. In a hot tub, though, low calcium is trouble. The water turns corrosive, pulling metal out of the heater and wearing the insides of pumps. In soft-water postcodes, calcium hardness needs adding at every refill.
The damage low calcium does
Water below roughly 80 ppm calcium hardness is undersaturated with respect to calcium carbonate. Given half a chance it will take calcium from wherever it can find it: tile grout, the heater element, the pump impeller, even the acrylic shell.
The effect is slow but it stacks up. Over a year of running soft water, an unbalanced tub can lose visible thickness from heater elements and develop pinhole corrosion in copper parts.
The feel changes too. Soft water is lovely in a shower but can seem slick or slimy in a hot tub when calcium is low.
The UK regions most affected
Typically soft: most of Scotland, the North West, North Wales, South West Wales, and pockets of Cornwall and Devon.
Typically hard: most of the South East, East Anglia, the Midlands, and the South coast.
Look up your water company's hardness map, often available as a postcode search. If your hardness sits below 60 mg/L as calcium carbonate, treat your area as soft for hot tub purposes.
Dosing calcium correctly
Buy calcium hardness increaser, which is calcium chloride flakes. Dissolve it in a bucket of warm tub water first, then add with the jets running.
For a 1,200-litre tub starting at 30 ppm, roughly 250g lifts you to about 200 ppm. Your brand's label gives a precise per-litre dose.
Add it at every refill. Calcium hardness barely shifts with use, so once dosed it holds for the whole refill.
FAQ
Could I just use bath salts?
No. Bath salts are sodium-based and do nothing for calcium hardness. They only add to TDS while leaving the real problem untouched.
Will added calcium feel harsh on skin?
At 100 to 250 ppm, no. That is the band bathers find comfortable. Hard tap water is often 400 ppm or more, well above the hot tub range.