How to look after your hot tub water and why


BISHTA’s main purpose since it was founded, has been to address issues of water hygiene and correct water hygiene management and maintenance. Today, BISHTA continues to take this role very seriously and it is very important that consumers ensure their hot tub is well looked after.

Why is this? A hot tub is one of the most wonderful, therapeutic and lifestyle-enhancing home improvements that consumers can invest in. However, it is important to ensure the water in your hot tub is clear, clean and hygienically safe, as some potentially harmful micro-organisms could otherwise cause problems if left unchecked. Therefore it is vital that your water is treated to BISHTA Standards. The good news is that this is neither expensive nor time consuming, and this factsheet aims to highlight this. 

There is a wide choice of water treatment products on the market which are safe, easy to use and reliable. The most common types of water treatment products are sanitisers (disinfectants), oxidizers, minerals and pH balancers. 


BISHTA recommends the continuous use of a residual sanitiser. A residual sanitiser is a substance which kills potentially harmful micro-organisms rapidly and leaves a residual in the water which can be measured. A residual sanitiser MUST be used, irrespective of any other back-up equipment or substance (i.e. an ozone generator or mineral cartridge) which may be used. 

The Health Protection Agencies (such as Health Protection England) and the Health and Safety Executive (HSE) recognise and consider the use of the following oxidising biocides essential: chlorine, bromine or PHMB (Biguanide) – see HSE/HPA Management of Spa Pools, Controlling the Risk of Infection. 


Chlorine is a fast and effective sanitiser, and its biocidal properties are well-established and proven. There are TWO DIFFERENT TYPES OF Chlorine granules which are available for use in hot tubs and are safe and easy to use. There is calcium hypochlorite which is stabiliser free and available in a quick dissolving form. Then there is sodium dichloroisocyanurate dehydrate (AKA Troclosene sodium, dehydrate or “dichlor”). This stabilised chlorine helps keep chlorine in the water on warm sunny days. Stabilised chlorine tablets (trichloro isocyanuric acid) are also available but some manufacturers of hot tubs state that their use will invalidate the warranty. 


Bromine (usually BCDMH) is also available. It is popular among hot tub owners as it does not give off the ‘swimming pool odour’ some associate with Chlorine, and it’s also very effective at higher temperatures. Bromine is sold in granule, lozenge, or tablet form. There is also a product available called sodium bromide, which produces Hypobromous Acid in the hot tub water, but it needs a catalyst such as chlorine or non-chlorine shock, to activate it. 


PHMB (also known as Biguanide) contains neither Chlorine nor Bromine. It destroys bacteria by assaulting the cell walls of the microbe. PHMB emits less odour than either Chlorine or Bromine and does not cause a smell at high temperatures found in hot tubs. NB: PHMB is not compatible for use with Chlorine or Bromine and correct advice must be sought from your BISHTA hot tub dealer if considering changing from one system to another! 

Sanitiser back-up products and systems 


Many hot tubs are fitted with ozone generators as standard, or available as options. Ozonators create ozone gas in a chamber and bubble it into the hot tub’s internal pipework. Ozone is a powerful oxidiser which kills bacteria as it passes through the ozonator. In order to use ozone, a hot tub has to have an ozonator installed in line. Irrespective of whether an ozonator is in use or not, a residual sanitiser maintained to prescribed levels must be used. 

Mineral Systems

Mineral systems, which utilise the bactericidal properties of metal minerals, can contribute towards keeping hot tub units free from contamination. They are dispersed through a floating dispenser, or sometimes via the filter cartridge. Some hot tub manufacturers also build a mineral dispensing canister into their hot tubs, into which special cartridges are placed. Minerals are released slowly and also contribute to a reduction in the consumption of Chlorine or Bromine, but, as with ozonators, must be used in conjunction with – not instead of – a residual sanitiser of chlorine or bromine. 


Oxidation is carried out utilising a “non-chlorine shock” which can break down some organic matter and help prevent/break down Combined Chlorine. However, monopersulphate is not a sanitiser (or disinfectant). Non-chlorine shock can be used for example before a party and again afterwards, bathing does not have to cease following the application. 

Super-chlorination using a quick dissolving calcium hypochlorite will both oxidise to help break down Combined Chlorine and will quickly kill bacteria. Shock dosing a hot tub at 20 ppm for 2.5 hours is recommended upon the initial fill of a hot tub and monthly thereafter. The residual chlorine needs to fall back to below 10 ppm before bathers can use the hot tub. A weekly shock dose at 10ppm with fast dissolving calcium hypochlorite granules is a good routine which can also be applied after a party when more people than usual have used the hot tub. 

The waterline should be kept clean and free from body oils, cosmetics, artificial tan and body lotions etc which can not only create a chlorine demand but a home for bacteria. Sodium bicarbonate is an easy means of doing this. 

Fast dissolving calcium hypochlorite is ideal for shock dosing a bromine treated hot tub but the calcium hypochlorite must be dissolved before coming into contact with the bromine. 

pH Balancers 

For your sanitiser to work most effectively, and for the comfort of bathers and protection of the hot tub’s equipment, the water’s pH must be controlled. pH is, in the most basic terms, the balance between acid and alkaline substances in the water. If water is too acidic, then the water will be corrosive, and if it’s too alkaline, then there is more likelihood for damaging scale to form. pH balancers are water treatment products which allow you to easily and safely control. pH balancers need to be added gradually to achieve the correct levels. 

Acids or pH reducer Used to lower a high pH. 

Alkalis or pH increasers Used to raise a low pH. 

T.A. increasers Used to increase the Total Alkalinity. Total Alkalinity is a measure of the total amount of alkaline substances in the water. If it’s too high, it will make an incorrect pH difficult to adjust, and if it’s too low, then small additions of either pH reducers or pH increasers will cause the pH to swing one way, then the other, rapidly. So the correct TA acts as a ‘buffer’ against this and allows you to control the pH easily, quickly and effectively. 


In order to know whether you are treating water correctly and, therefore, whether it is safe and hygienic to use, it is vitally important to test the water in your hot tub daily before use to find out what the pH and sanitiser levels in your water are. You will then need to make any adjustments required, as highlighted by your test results. 

There are a number of inexpensive test kits on the market which are quick, safe and reliable to use. Testing just takes a few seconds, whether by the tablet type test kit – where tablets which change colour according to readings are dropped into a sample of the hot tub’s water; or by test strips, small strips with impregnated pads which change colour when dipped into your hot tub’s water. NB: Both test strips and test tablets have use-by dates. It is important that you only use strips and tablets that are in-date. 

BISHTA recommended test reading parameters 

We recommend that your hot tub water is maintained daily according to the following levels. 


(if Bromine is used as your sanitiser): 

2mg/l – 4mg/l Domestic (Business 4 – 6 mg/l) 


(if Chlorine is used as your sanitiser): 

3mg/l – 5mg/l 


(if PHMB is used as your sanitiser): 



7.0 – 7.6 

Total Alkalinity: 

80 – 120 mg/l 

Draining down and refilling 

Hot tubs are very small bodies of water with relatively very high bathing loads. As water is used and recirculated, water treatment products and other particulates that aren’t removed by the filters build up over time, and there comes a point when even correct water maintenance regimes are applied, the water looks and feels “lank and lifeless”, and the only corrective action remaining is to drain the hot tub down and refill.

Hydropool's Water Purification Systems

If getting into a hot tub that is clean and safe everytime is important to you, Hydropool is now offering the most efficient water purification system in the world, combined with the Self-Cleaning hot tub and the Serenity hot tub filtration, making treating your hot tub water automatic.

1. Purify - Ozone
Water passes throught he patented Micro Cell.
Treats the hot tub water with Ozone (Natures Oxidiser)
This rapid oxidation reduces chemical usage by 50%

2. Expose to UV Light
The Ozone treated water then passes through a UV Chamber exposing it to a high volume of UVC light

3. Advanced Purification (1+1=3)
This combination of UV and Ozone creates the HydroClear PureWater System

4. Reduces...
Shocking your hot tub water
Our PureWater Systemeliminates 99% of all contaminants

5. Automatic...
PureWater System is on when your hot tub circulating
The Quick Diagnostics shows everything is in working order