Cal-hypo (calcium hypochlorite)
Also called: calcium hypochlorite, granular shock, pool shock.
Cal-hypo is calcium hypochlorite, a granular chlorine sold at about 50–73% strength and often bagged as pool shock. It dissolves fast and raises free chlorine quickly, but every dose also adds calcium hardness, and it raises pH.
Why it matters
The calcium is the baggage. In hard-water areas it builds up and can scale your surfaces, heater and salt cell. Cal-hypo has its place: a quick granular dose, or when calcium hardness is genuinely low and you want to raise it. As a daily sanitizer it slowly hardens your water. Read the percentage on the bag, because shock is sold anywhere from 47% to 78%. A cal-hypo shock can also briefly cloud the water on its own. For regular use, liquid chlorine adds only a trace of salt, so nothing accumulates.
In numbers
Roughly 19 ounces of 70% cal-hypo raises free chlorine by 10 ppm in 10,000 gallons, and adds calcium with it.