Why spas need shocking more than pools
Do the density math: two people in a 350-gallon spa equals about 100 people in a backyard pool — at 102°F, with jets aerating everything. Body oils and sweat accumulate faster than daily chlorine can oxidize, so the weekly shock isn't optional maintenance; it's the reset that makes the daily routine sufficient.
A worked example
A 350-gallon spa at 1 ppm needing a weekly shock to 10: a 9 ppm rise takes about 22 grams of dichlor — four and a half level teaspoons. After a crowded evening, the to-12 dose is ~27 grams. With MPS instead, a typical label calls for ~2 oz for this size — and the tub is usable again in a quarter hour.
The shock routine that works
Weekly: test, dichlor shock at night, jets on, cover off 20 minutes, then cover on and let it settle overnight. After parties: MPS immediately, dichlor if the water still smells the next day. And because dichlor adds stabilizer with every dose, spas on this routine still want their quarterly drain and refill — the daily dosing calculator covers the between-shocks routine. Foam that a shock doesn't clear isn't a dosing problem — the foam diagnostic finds the cause. And shock only works as well as the filter behind it: the filter cleaning schedule says when to rinse, soak, or replace.