Chlorine

• Chlorine is the best-known pool treatment and is used to kill algae, bacteria and other undesirable organisms. Chlorine isn't a substitute for good filtration, but works alongside the pool's filter to keep the water clean. Most chlorine formulations for pools include cyanuric acid to keep the chlorine from breaking down in the sun. The American Chemistry Council recommends monitoring chlorine levels on a daily basis to make sure the chemical is working correctly. Too much of this chemical can be irritating to mucus membranes and skin.
  
  

Bromine

• This chemical is similar to chlorine in that it works both as an oxidizer and a sanitizer, killing pathogens and algae. Unlike chlorine, bromine cannot be stabilized in sunlight and degrades after a few hours in an outdoor pool. This chemical has very little odor, however, and makes a good sanitizer for indoor pools and spas, where UV exposure isn't a problem.
  
  

Acids and Bases

• Swimming pools require an appropriate pH balance to keep the water clear and pleasant to swim in. The target pH is 7.4 to 7.6. If the pool is too alkaline, add muriatic acid, otherwise known as a 31 percent solution of hydrochloric acid. Sodium bisulfate also works for this purpose. For overly acidic pools, use sodium carbonate, otherwise known as washing soda.
  
  

Calcium

• Most pools also contain a certain level of dissolved calcium in the water. Too much calcium can lead to scaling and buildup on the pool and in the filtration system. There's no chemical you can add to reduce this -- just dilute the existing water with more water to reduce the amount of calcium. If your water contains a lot of calcium naturally, consider installing a water softener. If your pool doesn't have enough calcium, other elements in the water can corrode metal parts of the pool. Add calcium chloride to your pool to keep calcium levels up and buffer your pool's filtration system.