By The Zagosa Team. Published .
Reviewed or updated .
Total Dissolved Solids, or TDS, is a running total of everything dissolved in your water: sanitisers, hardness, alkalinity, sweat, sun cream, all of it. It climbs slowly and steadily, and once it passes a certain point no amount of dosing or balancing will bring the chemistry back under control. At that stage the only real answer is a refill.
What pushes TDS up
Every chemical you add. Every bather. Every top-up of mains water carrying its own hardness. Even the slow leaching from rubber parts adds a little.
Fresh UK mains water starts a tub at roughly 200 to 400 ppm TDS depending on hardness. Six weeks of regular use typically lands you at 800 to 1,200 ppm, and by twelve weeks the same tub can be sitting at 1,500 to 2,000 ppm.
The number that matters and how to read it
Above 1,500 ppm the water begins to feel slick or heavy. Above 2,000 ppm the chemistry stops behaving predictably and chloramine production speeds up.
A TDS pen costs about 15 to 25 pounds and gives an instant reading. Calibrate it once with a standard solution sachet and it stays accurate for months.
Test at every refill, then again at six weeks. That trend tells you whether you are on course for a 12-week refill or whether the tub needs attention sooner.
Refill or dilute
At 1,000 to 1,500 ppm you have room to plan. Book a refill in the next fortnight, no panic.
At 1,500 to 2,000 ppm, get the refill done this weekend.
Above 2,000 ppm, refill now. Shock and balancing chemicals are simply thrown away at this level, because the water can no longer hold them.
A half drain and refill roughly halves TDS. It is a handy stopgap if a full refill is awkward that week.
FAQ
Can a chemical bring TDS down?
No. Nothing on the consumer shelf removes dissolved solids. Reverse osmosis can, but the kit costs more than a year of refills.
Does soft fill water stretch the gap between refills?
A little. Soft water starts lower, so the climb from 200 to 1,500 takes slightly longer. The 12-week rule of thumb still holds for moderate use.