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A guide to hydrotherapy pool chemicals

Like any sort of swimming pool, a hydrotherapy pool will need to use chemicals to make sure the water is kept clean between users. It also helps to maintain the pH level of the pool to prevent any corrosion.

But which hydrotherapy pool chemicals do you need and why?

Why do you need hydrotherapy pool chemicals?

Treating hydrotherapy pool water with chemicals helps to keep it clean and enjoyable to use and swim in. It also helps with:

  • Keeping the water clear
  • Keeping the water clean and free from harmful bacteria
  • Preventing algae growth
  • Ensuring that the water is comfortable to be in
  • Preventing corrosion or damage to the pool
  • Stopping any bad smells or taste
  • Preventing scale formation in any elements of the pool
Hydrotherapy pool chemicals in various beakers and test tubes.
The amount of chemicals you add to your hydrotherapy pool needs to be carefully measured, as these are often added in very small doses.

What to measure in hydrotherapy pool water

There are four main things to measure in your hydrotherapy pool water; hardness, acidity, alkalinity, and the pH level.

The hardness of water may well depend on where you are based in the country; it refers to the amount of calcium and magnesium salts that are dissolved in your water. High levels of these salts can lead to cloudiness and scale formation, whereas low levels can cause corrosion to the surface of your pool.

Acidity in hydrotherapy pool water can be caused by carbonic acid, organic acids, sulphuric acid, and even nitric acid from pollution. However, with a hydrotherapy pool, we would expect these levels to be lower because your pool will be indoor.

The alkaline balance of your hydrotherapy pool is very important because you need it to outweigh the acidity, but only marginally. This includes the magnesium and calcium content in your water as described above.

Measuring the pH level of hydrotherapy pool water is key because it helps you keep an eye on how acidic or alkaline your pool is. It will also help you keep a healthy balance so that the water is comfortable for people to move around and swim in.

The pH scale is numbered 0 to 14 with 7 being neutral. For a stainless-steel hydrotherapy pool, the recommended pH level is 7.2–7.6, which is slightly more alkaline.

Sodium bisulphate or hydrochloric acid can be used to lower the pH level and the alkalinity of your hydrotherapy pool.

The pH and the alkalinity can be raised using sodium carbonate.

If you need to make your pool water harder, then calcium chloride is used.

A pipette dropping small amount of water on to a pH test.
Regular checks on the pH level of your water will help you work out the balance between the alkalinity and acidity of your water.

What chemicals are used in hydrotherapy pools?

The main chemical that is used in a hydrotherapy pool is a chlorine-based disinfectant. This will be put into your hydrotherapy pool using either a mechanical pump, slow-release feed, or simply by hand.

Chlorine compounds currently used in hydrotherapy pools include sodium hypochlorite solution, calcium hypochlorite granules/tablets, sodium dichloroisocyanurate granules, and others. These substances are measured in two ways in your pool:

  • Free chlorine residual: this is the amount of chlorine that is in your pool that has not yet reacted or been used to disinfect the pool. This should be measured daily using a testing kit and you should adjust the level of disinfectant in the pool accordingly.
  • Combined chlorine: this refers to the amount of chlorine that has already reacted and is no longer able to disinfect the water. If your hydrotherapy pool smells very strongly of chlorine (more so than usual), then it shows that there is a higher level of combined chlorine and more disinfectant needs to be added to the pool to level it out.

The total chlorine residual is the amount of free chlorine and combined chlorine together. Combined chlorine can only be determined once you have tested for both total chlorine and free chlorine (total chlorine – free chlorine = combined chlorine).


There are different things to look out for in hydrotherapy pool water to keep everything ticking along as it should be. In every instance, we urge you to double-check the amounts of chlorine and other components you are using to ensure that the balance of your pool is well maintained.


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