Pool Care /
What Is TDS in Pool Water Testing?
TDS means total dissolved solids in pool water: the dissolved minerals, salts, and other substances a test can detect, but not the exact cause of a problem.
Archive date: this article was written for PoolSkimmer on Oct 24, 2026 and given the earlier archive date of Jun 27, 2025 so the blog reads as an organized archive. It was not published on that date.
TDS in pool water testing: what it is
The term TDS refers to total dissolved solids. Put simply, it is the sum of all the material that remains in the pool water once you have taken out any visible debris. This encompasses minerals, salts and other kinds of dissolved matter. While a TDS figure can give you some insight into your pool water, on its own it will not tell you precisely what is in the water or if it is properly balanced.
The significance of a TDS reading
You should pay attention to TDS as an indicator of whether dissolved material has been accumulating. It can come from a number of places: the source water for filling the pool, chemicals put in over time, evaporation and refilling, or even bathers and the environment. When the numbers go up, the water tends to be more of a challenge to handle. A high TDS may be part of a bigger picture if you are also dealing with scale, cloudiness or trouble sanitizing.
Making sense of a high level
Do not make assumptions when you see a high TDS; let it prompt a closer look. You need to view it in context with your pH, hardness, alkalinity and chlorine results. TDS cannot by itself pinpoint if you have poor circulation, sanitizer loss or a buildup from fill water.
Here is how to approach it:
1. Go over your test history. An unexplained jump could be due to a change in maintenance, storm debris or an atypical top-off. 2. Assess clarity and your sanitizer. If the water is hazy or the chlorine is weak, there is likely a wider quality issue at play. 3. Give the filtration and circulation a once-over. Verify the skimmer basket is empty and the pump and filter are in good order. 4. Consider recent activity. Yard debris, rain and heavy use will have an impact. 5. Stick to the directions on the label and in the manual for any chemistry you put in.
Is it time to drain the pool?
A TDS reading alone does not dictate that. Some will factor it in when considering a water replacement, but there is no magic number that means you have to drain. It is a matter of the overall state of the water – can you still balance it? Is the clarity and sanitizer performance where it should be? If routine maintenance is not enough to clear up cloudy or unbalanced water, you would be better off having a pro take a look rather than fixating on the TDS.
Pitfalls to steer clear of
- Relying on TDS as the be-all and end-all of your testing.
- Thinking a high number tells you exactly what the contaminant is.
- Overlooking low chlorine or other readings that are out of range.
- Not accounting for the fact that source water will add to TDS with time.
- Failing to shut off the pump before you open the skimmer.
- Using chemicals in a way that does not conform to the product label.
Time to get a professional involved?
There is no harm in calling a qualified pool professional if you are unsure. They can put the TDS in perspective with the rest of the test data and put together a sensible plan for correction. Do so if:
- the TDS is on the rise and the water remains cloudy
- you are having difficulty stabilizing the chemistry
- your sanitation or filtration is underperforming
- you are thinking of replacing the water but do not know the root cause
When multiple factors are at work, a professional opinion is often worth having.
Sources and further reading
- [Hayward: Weekly pool maintenance](https://www.hayward.com/maintenance/weekly-pool-maintenance)