Answer:

To boost free chlorine, use some kind of chlorine "shock" that will raise the level and provide additional sanitization and oxidation. Quick dissolving chlorine compounds are ideal for a boost such as calcium hypochlorite, lithium hypochlorite or sodium hypochlorite (liquid chlorine). There are many kinds of chlorine shock and which form to use is up to you. There are advantages to each of the chlorine compounds such as price, speed at which it dissolves, affect on pH, etc.