Chemicals, Filter and Vacuum Use

• How often you need to change your pool water depends in large part on your daily maintenance routine. It's important to keep the free chlorine and pH levels at the right amounts. Chlorine should never drop below 1.0 ppm, or it will not be effective in killing algae and harmful bacteria. The pH levels of the pool should be between 7.2 and 7.6. Test your water daily. If you keep the levels steady, you don't have to change your pool water as often. Using a good filter and cleaning it out on a regular basis keeps your water clean, as does the use of an automated pool vacuum.
  
  

Hard or Soft Water

• The type of water you use in your pool can also be a factor in how often you need to replace it. Hard water contains more minerals, which build up on the walls and bottom of the pool over time. Scrubbing the walls on a regular basis with a brush or vacuum helps, but swimming pools that have hard water may still need to be emptied more frequently than those that have soft water.
  
  

TDS Levels

• As water evaporates from your pool, it leaves behind the minerals it contained. As you add more mineral-rich water, the ratio of minerals to water increases. A TDS test can measure how many minerals and salts your pool water contains. As this level increases, it takes more and more chemicals to clean your water. Once the TDS level reaches 1,550 ppm, it is time to change the water in your pool.
  
  

Other Considerations

• Some situations necessitate an immediate change of the pool water. Cloudy, green water, a bad smell, slimy pool walls and obvious debris are all signs that the pool water may be bad for a swimmer's health. While a chlorine shock may serve to clean the water temporarily, if the conditions are extreme, a complete change of pool water may be needed.