Manchester, Liverpool, Cumbria: soft water means low calcium, corrosive tendencies, and foaming. The balancing strategy is the opposite of the South East.
Soft water sounds easier and is in many ways. No scale, no kettle limescale, no fight to keep calcium hardness in check. The downside in a hot tub is that very soft water is corrosive and foamy. The chemistry strategy in the North West is essentially the inverse of the South East.
Calcium hardness in many North West postcodes is below 50 ppm in the mains. Below 80 ppm in the tub, the water becomes corrosive: it strips calcium from grout, heater elements, pump components, and even nearby tile.
Soft water foams more. Lower calcium reduces the surface tension of the water, so any surfactant (body oils, detergent on swimwear) creates more foam than it would in harder water.
The water can feel slick to the touch. Pleasant in the shower, slightly disconcerting in a hot tub.
Add calcium hardness increaser (calcium chloride flakes) at every refill. For a typical 1,200-litre tub starting at 30 ppm, you need around 250 to 300g to bring the level into the 100 to 200 ppm band.
Dose alkalinity to the upper end of the band (110 to 120 ppm). The buffer protects you against the natural tendency for soft water to drift to low pH and become more aggressive.
Keep pH on the upper end (7.5 to 7.7). The slightly more alkaline water reduces the corrosivity that low calcium causes.
Test calcium hardness monthly. If it has dropped (and it can, very slowly, as the water reaches equilibrium with surfaces), top up with more calcium chloride.
Foam is the constant background issue. Rinse swimwear before bathing. Soak the filter regularly. Use clarifier sparingly. Keep a defoamer to hand only as an emergency for guests.
Check the heater element annually for pitting or thinning, especially if the tub is older. Soft-water corrosion is slow but cumulative, and a heater that has run undersized on calcium for years can fail without warning.
Both are low in calcium, but rainwater has zero hardness and no chlorine. Never use rainwater as fill. The lack of any starting buffering is much worse than soft mains water.
At 100 to 200 ppm, no. That is well below the level that makes shower water feel scratchy. Hot tub bathers consistently prefer water in this band over very soft water.