Dichlor or bleach?
Dichlor is the standard spa chlorine because it dissolves instantly and adds a little CYA, which protects chlorine from the sun on an outdoor tub. But every ounce also raises CYA, and once CYA passes 30 to 50 ppm the chlorine gets sluggish. The usual fix is the dichlor-then-bleach method: dichlor for the first week after a refill, plain liquid chlorine or 6% bleach after that.
Shocking a hot tub
After a party or when the water smells, set the target to 10 ppm, add the dose with the cover off and jets on, and leave the cover off for 20 minutes so the gas can escape. Don't get back in until free chlorine is under 5 ppm, usually the next day.
A dose that looks tiny is right
At 400 gallons, 1 ppm of chlorine is about a quarter ounce of dichlor, roughly a teaspoon and a half. That's the whole point of measuring: a "capful" is three days' worth. Don't know your volume?Work it out here. For pH and alkalinity the pool pH and alkalinity calculators work at spa volumes too.