What chemicals are used to start up a pool while filling?

First off, test the water going into the pool to determine its "native" pH, chlorine level (municipal water usually has trace amounts), its alkalinity (its resistance to swings in the pH), & its calcium hardness. pH/alkalinity is first. Get the pH balanced, then add chlorine - liquid bleach is an Excellent shock treatment, also get some floater dispensers and plop some trichlor pucks or sticks in there. I recommend getting a Taylor test kit (pricey but critically accurate) for use once or twice a month, and a cheap little kit for daily use. I use borax (20 Mule Team at grocery store) for alkalinity issues, baking soda (also grocery store) for pH UP and muriatic acid (hardware stores) for pH down. Learning your waters native alk & pH is 3/4 the battle. Then just keep it chlorinated to kill the beasties. When you are adding new water or filling for the first time you should first add your metal out or metal preventer which ever is in your area to prevent staining,second adjust your alkalinity(sodium bicarbonate for up and acid for down) to between 80 120 ppm. Three adjust your(sodium carbonate up and acid down) PH 7.2-7.5 24 hours after this is done you can add your Chlorine 1.0 to 5.0 ppm 3.0 being ideal in a pool.Once a week use a non-chlorine shock to keep water sparkling.