Bob Lowry Breaks Down TDS in Pool Water

Total Dissolved Solids (TDS) is one of the most ignored numbers in pool water chemistry, even though it quietly influences how water behaves. TDS simply means the total amount of dissolved material in the water, measured in parts per million, and it is not the same as visible debris or suspended solids. Modern pool service pros usually measure TDS with a digital meter that reads conductivity and converts that to a ppm estimate. That convenience is why it exists, because the old-school “total dissolved residue” method required evaporating a carefully measured sample and weighing what was left. The key takeaway for pool maintenance is that TDS rises as you add chemicals over time, and that rising baseline changes the way your test results and balance calculations perform.

A practical way to think about TDS is the “kettle scale” analogy: keep boiling water away and you eventually see what was dissolved all along. In pools, the biggest contributors are often chloride and sodium. Every time you add chlorine, it ultimately becomes chloride. Many common pool chemicals begin with sodium, and those sodium ions stay in the water. Add sodium hypochlorite, sodium bicarbonate, soda ash, or even sodium chloride, and you are stacking more dissolved solids into the same body of water. Some dissolved materials do not ionize well, so a conductivity-based TDS meter can undercount certain additions, but it is still a strong day-to-day indicator for tracking the overall “stuff load” in the water.

TDS becomes especially important in saltwater pool care. A salt chlorine generator may require roughly 3,000 to 3,400 ppm of salt to operate, and your fill water might start around 300 to 500 ppm TDS. Add routine chemical maintenance and it is easy to land in the 5,000 to 6,000 ppm range when you include salt. That is still only about half of 1% by weight, but it is enough to change water behavior and it must be accounted for in saturation index calculations such as LSI. Just as important, the saturation index is about scale forming or corrosive water with respect to plaster and calcium carbonate equilibrium. It does not predict metal corrosion, which is driven by factors like water velocity, dissolved ions, and real-world exposure to concentrated salt as water splashes out and evaporates, leaving salt behind on decks, furniture, and equipment.

So when is TDS “too high” in a non-salt pool, and what problems show up? A useful guideline is a maximum of about 1,500 ppm over the starting TDS, because that signals the water has accumulated a lot of untracked dissolved byproducts. High TDS can show up as brackish taste, persistent tint or color even when the water is clear, and strange issues like poor chlorine performance or “we tried everything” situations. Some of these are color bodies that do not bleach easily and are too small for standard filtration, which is why advanced options like ozonation, UV support, and better surface removal can matter. A simple habit helps: track TDS across every pool, and treat the meter like a long-life tool on your truck, because knowing the trend can be the difference between chasing symptoms and making the right call on dilution, draining, and targeted treatment.