Total dissolved solids

TDS is everything dissolved in the water added up: salt, calcium, stabilizer, the leftovers of every chemical anyone has ever added. It only goes up, and it only comes down when water is replaced.

What it's actually good for

TDS on its own rarely tells you to do anything. It's a summary. High TDS means high something-else, and the something-else is what you act on.

Where it earns its place is as a drain indicator. When TDS has climbed a long way above your fill water and the pool is getting harder to keep balanced, that's the water telling you it's near the end of its useful life.

Salt pools read high on purpose

3,000 ppm of salt is 3,000 ppm of TDS. A salt pool reading 3,500 is normal, not a problem, and it is not a reason to drain. Judge a salt pool by its salt reading and how it's behaving, not by TDS. See salt.

How often to test

Quarterly, or when something isn't adding up. TDS moves slowly. Testing it weekly gives you last quarter's number with extra steps, and a copied-forward reading is worse than a blank one because it looks like you measured.

TDS and LSI

TDS is in the LSI calculation, but its weight is small. Going from 1,000 to 4,000 ppm shifts the index by about 0.1. It's included for completeness rather than because it drives the result. pH, calcium and alkalinity do the real work. See reading the LSI.