Water Chemistry /
What Causes Total Dissolved Solids to Build Up in Pool Water?
TDS builds up from evaporation, fill water, chemicals, swimmers, and debris. Learn the common causes, warning signs, and when to drain or call a pro.
Archive date: this article was written for PoolSkimmer on Oct 7, 2026 and given the earlier archive date of Aug 30, 2025 so the blog reads as an organized archive. It was not published on that date.
Understanding total dissolved solids in pool water
By way of definition, total dissolved solids (TDS) are the mix of minerals, salts, metals and byproducts from your treatments that have dissolved into the pool water and are not going anywhere. You can skim out a leaf or some dirt, but TDS is another matter; it lingers and will build up as water is cycled out and put back in.
Some TDS is to be expected in any swimming pool. The trouble comes when the level gets too high, making for dull water, scale or corrosion and complicating the task of keeping the water in balance.
What drives TDS levels up
The fundamental cause of rising TDS is that while water may leave the pool, the dissolved material does not. There are several ways this happens:
- *Evaporation* will concentrate what is already there.
- Refilling the pool puts in whatever is in your source water, be it hardness minerals, sodium or metals.
- *Sanitizers* leave behind their own byproducts once they have done their job.
- Chemical adjustments for pH or alkalinity add to the dissolved content with every correction.
- In hard-water regions, calcium chemicals can boost mineral levels.
- Then there is the swimmer load – lotions, sweat, cosmetics and body oils.
- *Debris* and organic matter will eventually break down into smaller compounds.
- Even rain and runoff can bring in soil and fertilizer.
Owners of saltwater pools should note the salt itself figures into the baseline TDS, so the numbers will run higher than in a fresh-water setup.
Why TDS is always on the rise
Pool maintenance is an additive process. Every time you make an adjustment you put something in the water and it does not just vanish without dilution. Be it clarifiers, algaecides, stain control or the residue of normal filtration, these things accumulate. It is why a pool can appear spotless yet still register an increase in TDS.
Indicators of high TDS
You may not see an immediate problem with elevated TDS, but it will make itself known in the chemistry and clarity of the water.
Keep an eye out for:
- a flat, unsparkling look to the water
- cloudiness that will not go away
- scale on your equipment or tile
- sanitizer that is not performing as it should
- more volatility in your balance
- signs of corrosion
Of course, any of these could be due to other chemistry issues. Before pointing the finger at TDS, verify your pH, alkalinity, calcium hardness and sanitizer levels.
Slowing the buildup
While you cannot stop TDS from forming, you can keep it in check.
Make a habit of testing to prevent overcorrecting. Put a cover on the pool to cut down on evaporation where you can, and use the best quality fill water available in your area. Be judicious with your chemicals and read the labels. Clear out the skimmer and pump baskets (make sure the pump is off before you open the skimmer) and get rid of leaves before they decompose. And as with any chemical work or equipment cleaning, don’t forget your PPE and the manufacturer’s instructions.
To drain or call in help
When TDS is on the ascent and the pool becomes a chore to balance, a partial drain-and-refill is often the only sensible option to bring it down, particularly if your source water is heavy with minerals.
It is worth calling a pool professional to see if the TDS is truly the culprit or if something else is at play. Contact a service if you are dealing with well water, repeated imbalances, abnormal equipment performance or any sign of an electrical fault. If the water is unsafe or visibly contaminated, do not swim.
In short
TDS is a fact of life in pool water because you are constantly adding new material from swimmers, debris and chemicals while evaporation takes only the water. Do not try to treat it like a one-off chemical issue. The answer is consistent upkeep, proper testing and knowing when to dilute.