kalk reactors are usually used for top-off water, its pumped through the reactor then into the tank (or sump)
kalk reactors just use a spinning stirrer or a pump to mix the kalk and water. you need to add new kalk to the reactor around once a week depending on how much top-off water you use, and how well it mixes it up.
kalk will raise the Ph of makeup water to about 12.5, which will help counteract the normal tendancy for tanks to lower Ph overtime. kalk also will "lock up" phosphates in your top-off water, and help prevent algae blooms. Many people run kalk reactors at nighttime with a dosing pump to prevent the nightly Ph swings most tanks encounter.
calcium reactors use acidic C02 gas to dissolve calcium carbonate into the water.