If you’ve ever stared at a “perfect” test strip while the pool still acts weird, you’re probably missing one of the quietest drivers in pool water chemistry: TDS. We dig into what Total Dissolved Solids actually means, why a simple digital meter belongs on every service truck, and how TDS can swing your saturation index enough to change your real-world results. Along the way, we unpack the old-school truth behind TDS, what the meter is really reading (conductivity), and why it is still a valuable approximation for everyday pool maintenance decisions.

We also get specific about what’s building up in the water over time. Chlorine doesn’t just disappear, it becomes chloride. Many of the products you add carry sodium, and those ions stay behind. That matters even more in a saltwater pool, where you might be running 3,000 to 3,400 ppm salt before you count everything else. We talk through why a 5,000 to 6,000 ppm TDS reading can be normal with a generator running, but can turn into a chemistry headache when the cell is off and the salt is just along for the ride.

One of the biggest takeaways is a myth-buster: the saturation index can describe scale forming or plaster corrosivity, but it does not predict metal corrosion. Real corrosion shows up in places you can see, like salt splash-out that dries into concentrated salt on decks and equipment, or motors and hardware that fail even when the index looks “fine.” We also cover when high TDS is a drain-and-dilute conversation in non-salt pools (about 1,500 ppm over starting), plus the tricky world of color bodies, organic staining, and why chlorine alone sometimes cannot fix yellow or tinted water.

If you want fewer mystery problems and faster diagnoses, subscribe, share this with a pool pro friend, and leave a review so more techs start testing what actually builds up over time.

We break down what TDS really measures, why it affects the saturation index, and why so many pool pros still ignore it. We also clear up a costly myth: “balanced” water can still corrode metal, especially around saltwater pools.  
• why TDS testing fell out of routine pool service and why it matters  
• what a TDS meter actually measures through conductivity and why it is an approximation  
• what builds TDS over time, especially chloride and sodium from common pool chemicals  
• how saltwater pools push TDS higher and what happens when the generator is off  
• why the saturation index describes plaster risk, not metal corrosion risk  
• real-world clues of high TDS such as brackish taste, stubborn color, and “can’t fix it” water  
• how color bodies and organics can stain plastics and cleaners and resist chlorine  
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