What Is TDS in Spa & Swim Spa Water? Why It Matters

What Is TDS in Spa & Swim Spa Water? Why It Matters

Your spa water might look crystal clear, but did you know it can still contain a growing concentration of dissolved substances that affect water quality?

One of the most overlooked measurements in spa and swim spa maintenance is Total Dissolved Solids (TDS).

While most spa owners are familiar with chlorine, bromine, pH and alkalinity, TDS is another important indicator of your water's overall condition. Understanding what TDS means, why it increases and when it is time to replace your water can help you avoid ongoing water-quality problems.

At Pool & Spa World Tasmania, we regularly help customers understand their spa water chemistry. Here's what every spa and swim spa owner should know about TDS.

What Are Total Dissolved Solids (TDS)?

Total Dissolved Solids, commonly referred to as TDS, is a measurement of the combined concentration of dissolved substances in water.

These substances can include:

  • Minerals such as calcium and magnesium

  • Dissolved salts and metals

  • Residues from spa water-care chemicals

  • Dissolved substances introduced by bathers

  • Minerals already present in your original fill water

TDS is measured in parts per million (ppm), which provides an indication of the concentration of dissolved material in your spa or swim spa.

Every time you add chemicals, top up your water or use your spa, the composition of the water changes.

Over time, the concentration of dissolved solids can increase, even when the water appears clean.

Importantly, a TDS reading does not tell you whether your spa water is sanitised or safe to use. It measures dissolved substances, not bacteria or sanitiser effectiveness directly.

Why Do TDS Levels Increase in Spa and Swim Spa Water?

Unlike a swimming pool, a spa generally contains a relatively small volume of water that is heated and used regularly.

These conditions make maintaining water quality particularly important.

Several everyday activities contribute to increasing TDS levels.

1. Adding Spa Chemicals

Every chemical treatment changes the chemistry of your water.

Sanitisers, pH adjusters, alkalinity increasers and other water-care products can contribute dissolved substances or leave dissolved residues behind.

This doesn't mean you should avoid treating your water. Correct chemical treatment is essential for maintaining a hygienic spa.

However, it explains why water that has been maintained for several months may have a higher TDS reading than when it was first filled.

2. Bathers Using the Spa

Every person entering a spa introduces substances such as perspiration and residues from skincare products.

Some of these substances dissolve into the water, while others remain as oils or particles that require filtration and oxidation.

Showering before entering the spa can help reduce the amount of contamination introduced.

3. Water Evaporation

Spas and swim spas operate at elevated water temperatures, encouraging evaporation.

When water evaporates, most dissolved minerals and salts remain behind.

As fresh water is added to replace the evaporated water, additional dissolved minerals may be introduced.

Over time, this process can gradually increase the overall concentration of dissolved solids.

4. Your Original Water Supply

Not all freshwater supplies have the same TDS levels.

Water naturally contains dissolved minerals, and the starting concentration can vary depending on the source.

This is particularly relevant for Australian spa owners using bore water, tank water or other alternative water supplies.

Your original fill-water TDS provides a useful baseline for monitoring how much your spa water changes over time.

What Should the TDS Level Be in a Spa or Swim Spa?

One commonly used industry guideline recommends replacing spa water when the TDS reading has increased by approximately 1,500 ppm above the original source-water reading.

For example:

Water measurement TDS reading
Original fill water 250 ppm
Spa water after regular use 950 ppm
Increase above original water 700 ppm
Example replacement threshold 1,750 ppm

In this example, a reading of 950 ppm would not automatically indicate that the water needs replacing based on TDS alone.

However, if the reading reaches approximately 1,750 ppm, it meets the commonly used 1,500 ppm increase guideline for water replacement.

Remember: This is a general water-replacement guideline, not a guarantee of water safety. Water may need to be replaced earlier due to heavy usage, contamination, poor water quality or manufacturer recommendations.

Salt-based systems also require special consideration because the salt deliberately added to the water contributes to TDS.

What Happens When TDS Gets Too High?

A high TDS reading is a useful warning that the water has accumulated dissolved substances.

It does not necessarily identify which substances are present or prove that they are harmful.

However, ageing spa water can experience several issues.

Water Becomes Difficult to Balance

Have you ever found yourself repeatedly adjusting pH or alkalinity, only to find the water chemistry is still difficult to maintain?

This can be a sign that the water requires closer investigation.

High TDS may accompany changes in water chemistry and accumulated contaminants, although TDS alone is not necessarily the cause.

Reduced Water Clarity

Cloudy or dull-looking water can be associated with accumulated contaminants, poor filtration, inadequate sanitation or incorrect chemical balance.

High TDS can be another indication that the water is ageing.

Increasing Chemical Demand

As contaminants accumulate, maintaining an appropriate sanitiser residual may become more difficult.

You might notice that your spa requires more frequent treatment than previously.

Although the dissolved solids themselves do not all consume sanitiser, a rising TDS reading can accompany the accumulation of substances that affect water treatment.

Potential Equipment Concerns

Depending on the specific minerals and salts present, water with an unsuitable chemical balance can contribute to scale formation, staining or corrosion.

Correct pH, alkalinity, calcium hardness and overall water balance remain essential for protecting your spa components.

Can You Lower TDS Without Draining Your Spa?

This is one of the most common questions spa owners ask.

For most residential spas and swim spas, replacing water is the most practical method of reducing accumulated TDS.

Standard cartridge filters remove suspended particles and debris but cannot remove most substances already dissolved in the water.

Similarly, adding more sanitiser, oxidiser or clarifier does not generally remove dissolved salts and minerals.

Specialised water-treatment technologies can remove some dissolved substances, but these are not normally part of a standard residential spa filtration system.

A partial water replacement can reduce TDS, provided the replacement water has a lower TDS reading.

For smaller spas, a complete drain, clean and refill is often the most practical option.

How Often Should You Drain and Refill Your Spa?

There is no single replacement schedule suitable for every spa.

The required frequency depends on:

  • Spa water volume

  • Number of people using the spa

  • Frequency of use

  • Original water quality

  • Water chemistry and maintenance history

  • Manufacturer recommendations

As a general maintenance starting point, many residential spa owners plan a water change approximately every 3–4 months, although heavily used spas may require more frequent replacement.

Larger swim spas may have different maintenance and water-replacement requirements because of their greater water volume and individual treatment systems.

Rather than relying on the calendar alone, regular water testing and monitoring of water quality can help determine when replacement is necessary.

How Do You Test TDS in Spa Water?

TDS is commonly measured using a digital TDS meter or suitable professional water-testing equipment

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