Simply put, you don’t want phosphates in your spa water. They will impair the ability of your primary sanitizer to kill bacteria & other pathogens. They may also manifest as foaming in your spa.
Phosphates are measured in extremely small concentrations. They are measured in ppb (parts per billion) rather than ppm (parts per million) as with all other readings for your spa water.
Ideally you want no phosphates (0 ppb), however measurements below 100 ppb are generally not going to cause an issue. Anything over 500 ppb is definitely problematic and requires removal of the phosphates using a Phosphate Removing product.
Most municipal water will have little to no phosphates. However this may not be true in agricultural areas (phosphates are also fertilizers) or areas with high metal content in the water as some agents used by cities to remove metals also contain phosphates.
If your water source is a well and/or you are in an agricultural area, you should check for phosphates.
