Pool or spa water that is chemically treated and properly balanced will contain a certain amount of saturation from soluble salts in the water. This is known as Total Dissolved Solids (TDS). Everything added from the chlorine, balance chemicals and contaminants from swimmers and the environment leads to high TDS.

A High TDS can lead to water quality problems such as:

• Increased chlorine consumption
• Scale formation
• Cloudy pool water
• Corrosion of metals causing discoloration of metal parts in water
• Continual algae problems
• A salty taste to the water
• Irritation of eyes and skin

Evaporation is the biggest contributor to high TDS in a pool or spa. In spas, TDS builds rapidly due to the higher water temperature. When water evaporates only pure water leaves to the air. All solids stay behind in the body of water of the pool or spa. The evaporation rate of pools in Southern California and Arizona ranges from 6 to 8 feet per year. So, an average backyard pool loses all the water to evaporation in one year. Whenever you add more water to the pool to top off what was lost from evaporation you are also adding more TDS to the pool from the fill water. When you add chlorine, shock products, balance chemicals or algaecides you are increasing the TDS. When you add stabilizer cyanuric acid you are also increasing the TDS. If you use Tri-Chlor tablets, then you are adding cyanuric acid every day to the water that will lead to increased TDS. 1lbs of Tri-Chlor in a 10,000-gallon pool increases the TDS by 16 ppm.

As all these solids continue to accumulate, the water reaches a point where it can no longer work as a solvent to dissolve solids. High TDS in a swimming pool is a good indicator of old worn out pool water. The water is just plain tired. The only way to wake up and recharge the pool and lower TDS is to do some draining and diluting. You don’t need to drain the entire pool but a partial drain or even regular monthly draining of a foot or two of water can help. As you refill and dilute the water will be more fresh and pure. The ideal TDS levels for a swimming pool are between 1000 to 2000 ppm. In spas the ideal level is 750 ppm to 1500 ppm. The SGS SLTD-100 Salinity / TDS Meter gives very accurate Total Dissolved Solids measurements. It should be tested at least twice a year. 

It is important to also test and know the TDS of your tap water. Once TDS in the pool or spa reaches 1500 ppm over the tap then it will take 50% more chlorine to sanitize the pool. This is why testing and keeping TDS managed can help keep chemical costs down and give you a better swimming experience. Regular draining and diluting can help keep your TDS down and save you from a salty situation in your pool.