Seattle Public Utilities delivers some of the softest municipal water in the country: roughly 22–27 mg/L of hardness (about 1.5 grains) and alkalinity between 15 and 25 mg/L. For drinking, that is excellent. For a hot tub, it is corrosive. Water that low in dissolved minerals pulls calcium out of whatever it touches — the heater element sheath, the ceramic seal faces in the pump, the brass in the unions.
The four numbers
Calcium hardness: 150–250 ppm. Straight from the tap you are at 25. Calcium hardness increaser is the single most important thing you add to a Seattle fill, and most owners have never heard of it because inland advice is all about reducing hardness.
Total alkalinity: 80–120 ppm. This is the buffer that keeps pH from swinging. Seattle water has almost none, which is why a freshly filled tub reads fine on day one and is at pH 8.4 by the weekend. Set alkalinity before pH, always.
pH: 7.4–7.6. Only after alkalinity is set. Chasing pH first in unbuffered water is a losing game.
Sanitizer: per the label. Chlorine or bromine at the normal range; soft water does not change this part.
What goes wrong when the numbers are ignored
The heater element etches. Current leaks to ground, the GFCI trips, and the element is replaced — and then replaced again two years later if the water is still unbalanced. Pump shaft seals weep. Jet inserts wear loose. None of it is dramatic, all of it is expensive over a decade, and all of it is preventable with a jar of calcium increaser and a test strip.
Renton is the exception
Renton pumps its own groundwater at about 4 grains — still moderate, but enough that we descale rather than add calcium there. If you are on a Renton address, the fill needs less help.
Questions
Do I need a water softener for a hot tub in Seattle?
How often should a Seattle tub be drained?
Related services
Next note: Hot tub not heating? Why it is usually flow, not the heater