Dichlor, then bleach
Dichlor is convenient but adds about 0.9 ppm of stabilizer (CYA) for every 1 ppm of chlorine. A little stabilizer makes chlorine last longer; too much makes it sluggish. So use dichlor for the first week or two after a refill, until CYA reaches about 30 ppm, then switch to plain bleach or liquid chlorine, which add none.
Public-health guidance (CDC) advises against stabilizer in hot tubs because it slows disinfection. The dichlor-then-bleach method keeps it low on purpose, about 30 to 40 ppm, where the chlorine the stabilizer asks for (7.5% of CYA) is still no more than 3 ppm. If you prefer none, use bleach only from a fresh fill and test before every soak. Either way, free chlorine must be at least 3 ppm.
After a soak
Each soak uses chlorine. Two people soaking for 20 minutes in a 400 gallon tub use roughly 1.1 to 3 ppm, depending on temperature and how clean the soakers are. Test, then add enough to get back to your target. In a 400 gallon tub, 3 ppm is 0.29 oz (8.1 g) of dichlor.
Per 1 ppm, by tub size
| Tub | Dichlor 56% | 8.25% bleach | 10% liquid chlorine |
|---|---|---|---|
| 200 gal | 1.4 g | 8.6 mL | 7.6 mL |
| 300 gal | 2 g | 13 mL | 11 mL |
| 400 gal | 2.7 g | 17 mL | 15 mL |
| 500 gal | 3.4 g | 21 mL | 19 mL |