Total Dissolved Solids

TDS – a build up of materials that reduce chlorine effectiveness.

The dissolved solids concentration in water is the sum of all the substances,
organic and inorganic, dissolved in water.
This also is referred to as “total dissolved solids”, or TDS.
Calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate,
and silica typically make up most of the dissolved solids in water. USGS

Typical backyard pools and spas should not exceed around 1,500 ppm.
(Salt pools will be higher and require a different standard.)
Excessively high TDS will render poor chemical adjustment results.

When both steam or evaporation occur on swimming pools or spas the TDS level will rise.
Steam and evaporation are pure water vapor.
These carry away only pure water leaving behind higher concentrations of contaminants.

Draining all, or a portion, of the pool or spa and refilling with fresh water is currently the best way to resolve this problem.