Total Dissolved Solids (TDS) is the total concentration of all dissolved minerals, salts, and chemical compounds in your pool water. Every chemical you add to the pool — chlorine, pH adjusters, shock, alkalinity increaser, algaecide — leaves dissolved residue behind when the active ingredient is consumed. Over time, these accumulate.
Evaporation makes the problem worse. When water evaporates, only pure water molecules leave — everything dissolved stays behind in a smaller volume, concentrating the TDS further.
| TDS Level | Status |
|---|---|
| Below 1,500 ppm | Excellent |
| 1,500–2,000 ppm | Good — monitor annually |
| 2,000–2,500 ppm | Acceptable — consider partial drain next season |
| 2,500–3,000 ppm | High — plan partial drain |
| Above 3,000 ppm | Problem — chemicals becoming inefficient, drain needed |
Note: Saltwater pools have inherently higher TDS (3,000–5,000 ppm from the salt itself) — the high TDS threshold does not apply to salt pools. TDS meters on salt pools read salt + other dissolved solids together.
Drain 25–30% of the pool and refill with fresh water. A 25% drain reduces TDS by approximately 25%. For a pool at 3,000 ppm, a 33% drain brings it to approximately 2,000 ppm. Rebalance all chemistry after refilling.
A complete drain resets TDS to your fill water's level. Most tap water is 100–400 ppm TDS, giving you a fresh start. Best time to do a full drain: spring before swim season opens. Consult with a pool professional before draining a fiberglass pool (risk of float due to groundwater pressure).
A mobile RO service parks a truck at your pool, connects to the water, and filters it through reverse osmosis membranes. This removes TDS, CYA, calcium, and metals without draining the pool. Typically costs $300–$600 for a residential pool but saves the cost of refilling and rebalancing from scratch.
TDS accumulates slowly — most pool owners only need to think about it every 2–3 years. Test TDS at the pool store during your monthly water test. Do not pay for TDS additives — no chemical reduces TDS. Only dilution works.
PoolLens lets you log monthly pool store test results including TDS. See the trend over seasons and know exactly when it is time for a partial drain.
Open PoolLens Free →Pool TDS below 1,500 ppm is ideal. Most pools function well up to 2,000 ppm. Above 2,500 ppm, chemical efficiency decreases and water may appear dull. Saltwater pools have inherently higher TDS from salt and are evaluated differently.
Every chemical you add leaves dissolved mineral residue. Evaporation concentrates these over time since only pure water leaves — dissolved solids stay. After years of topping off without draining, TDS accumulates significantly.
Yes. Very high TDS (above 3,000 ppm in non-salt pools) causes water to appear dull or flat despite correct chemistry. Chemicals become less effective, requiring higher doses. Swimmers may notice a "heavier" feeling in the water.
In a typical residential pool, a partial drain (25–30%) every 2–3 years is usually sufficient. In heavily used pools, hot climates, or pools using many chemical additives, annual partial drains may be needed. Test TDS annually at the pool store.
Not directly — but high TDS reduces the effectiveness of chlorine and other sanitizers, which makes algae easier to establish. Additionally, very high TDS water is "tired" water that does not respond predictably to chemical additions, making maintenance harder.