Understanding and Managing Total Dissolved Solids (TDS)

In the world of pool chemistry, we spend a lot of time focusing on specific parameters like chlorine, pH, and alkalinity. However, there's a broader measurement that encompasses almost everything in your water: Total Dissolved Solids, or TDS. While not a number you need to adjust weekly, understanding TDS is crucial for the long-term health of your pool, especially for saltwater pool owners. This guide will demystify TDS, explaining what it is, where it comes from, and how to manage it.

What is Total Dissolved Solids (TDS)?

TDS is the measurement, in parts per million (ppm), of the total amount of all dissolved substances in your pool water. This includes everything that isn't pure H₂O. Think of it as the sum of all the minerals, salts, and chemical residues you've added over time.

Common components of TDS include:

• Salts: In a saltwater pool, sodium chloride is the largest contributor to TDS. This is expected and necessary.
• Balancing Chemicals: Everything you add leaves something behind. This includes calcium from Calcium Hardness increaser, bicarbonates from Alkalinity increaser, sulfates from pH decreaser (dry acid), and cyanurates from stabilizer.
• Source Water Minerals: Minerals naturally present in your tap water, like calcium and magnesium.
• Organic Matter: Dust, pollen, bather waste, and other debris that has broken down.

As you can see, TDS is destined to rise over time in any pool as chemicals are added and water evaporates, leaving the solids behind.

How TDS Affects Your Pool

For a long time, the exact effects of TDS were debated, but it's now clear that high TDS levels can cause several problems, especially in saltwater pools.

1. Reduced Chlorine Efficiency

High TDS can make your chlorine less effective. The dissolved solids can interfere with the chlorine's ability to oxidize and sanitize, meaning you may need to maintain a higher free chlorine level to achieve the same killing power. This puts more strain on your salt cell.

2. Corrosion of Equipment

Water with high TDS becomes more electrically conductive. This can accelerate galvanic corrosion, a process that can damage metal components in your pool like heater elements, ladders, and light rings.

3. Staining and Scaling

When TDS levels get too high, the water becomes "saturated," and solids can precipitate out of solution. This can lead to cloudy water and the formation of scale on your pool surfaces and, crucially, on your salt cell plates. This scaling reduces the cell's efficiency and lifespan.

4. Inaccurate Salt Readings

As discussed in our guide on reading discrepancies, salt chlorine generators estimate salt levels based on conductivity. High TDS from sources other than salt increases conductivity, which can trick your system into thinking there's more salt than there actually is, leading to an inaccurate reading and potentially insufficient chlorine production.

Ideal TDS Levels and Testing

TDS is measured with a special digital meter that is different from a standard salt tester. The ideal range depends on your pool type:

• Traditional Chlorine Pools: The ideal TDS is below **1,500 ppm**.
• Saltwater Pools: This is the crucial difference. Since salt is a major component of TDS, the baseline is much higher. A saltwater pool's TDS will be its salt level plus all other dissolved solids. For example, if your salt level is 3,200 ppm (which you can calculate with our main calculator), your TDS will be at least that high. The recommended maximum TDS for a saltwater pool is generally **1,500 ppm *above* your salt level**. So, if your salt is at 3,200 ppm, you should try to keep your total TDS below 4,700 ppm.

You should test TDS a few times a year, or if you start to experience issues like persistent cloudiness or trouble balancing your water.

How to Lower TDS

This is the simple, yet often undesirable, answer: **there is no chemical that can lower TDS.** Products marketed as "TDS reducers" are flocculants that clump some solids together for filtration but do not remove what is truly dissolved.

The only effective way to lower TDS is through **dilution**. You must drain a portion of your pool's water and replace it with fresh water that has a low TDS level. This is often referred to as a "partial drain and refill."

For example, if your TDS is at 6,000 ppm and your target is 4,500 ppm, you would need to replace roughly 25% of your pool's water. This process will also lower all your other chemical levels (salt, CYA, calcium, etc.), which will then need to be re-balanced. This is why managing TDS is a long-term strategy, aiming to slow its accumulation to avoid frequent draining.

While TDS might seem like a complex topic, understanding it is a key part of advanced pool care. By being mindful of the chemicals you add and knowing when a partial drain is necessary, you can ensure the long-term health and clarity of your pool water.