Chlorine or Bromine in a Hot Tub?


One of the most common questions we get asked from spa and hot tub owners is which is the best sanitiser to use – Chlorine or Bromine? There are many theories out there as to which is better, however we decided to gain firsthand experience and run our test hot tub on both in order to provide you with a practical answer. The conclusion we came to is that both are perfectly suitable sanitisers, however bromine does boast three crucial advantages over chlorine, which we believe makes it a more suitable sanitiser for hot tubs.

Less odorous – Running your spa on chlorine can create a strong chlorine smell in the water even when the chlorine level is kept within the recommended range of 2-4 ppm. Although this is perfectly normal it can result in an unpleasant bathing experience. Bromine is less odorous than chlorine and as a result this smell doesn’t exist and a more pleasant bathing environment can be enjoyed.

Effective at a broader pH range – The recommended pH level for a chlorine spa is 7.2 – 7.6. This is because the effectiveness of the chlorine as a sanitiser significantly decreases at higher pH levels. Bromine on the other hand, is an active santiser at pH levels of 8.2, therefore the pH level can be maintained at a much broader range of 7.2-8.2. This allows more flexibility when controlling the pH level and is particularly beneficial for hard water areas where the incoming water supply naturally contains high pH and alkalinity levels.

Slower dissolving – Regardless of which sanitiser you use, both are available in tablet form and ideally should be used in conjunction with a floating dispenser. The main difference we experienced is that bromine is much slower dissolving than chlorine and as a result it is a lot easier to maintain the correct sanitiser level in the water. This is crucial in order to reduce the amount of work involved in maintaining your hot tub.

To make this a fair argument it is only right that we look into the disadvantages of using bromine over chlorine. As bromine is slow dissolving it can have difficulty in recovering an adequate concentration after the hot tub has been heavily used or is started up after being emptied. In this case you need to add another sanitiser such as oxy shock or stabilised chlorine granules just to boost the level. Another disadvantage is price, as bromine is a little more expensive than chlorine.

Despite these drawbacks we firmly believe that the advantages of using bromine over chlorine far outweigh the disadvantages. As a result we would always recommend using bromine as opposed to chlorine in a hot tub.

Summer’s coming to an end – what to do next?

With the school holidays over and autumn on the horizon, many pool owners will start thinking about closing their pools. The good news is summer doesn’t appear to be going without a fight as forecasters are predicting prolonged spells of warm weather for some parts of the UK over the next couple of weeks. Therefore you may want to put any plans to close your pool on hold for a bit longer….

When you decide that the time has come to close your pool for the winter then this blog post is designed to give you a helping hand. If you have an in-ground pool then it is strongly advised that you winterise your pool. If you have an above ground pool then you can also winterise your pool or you have the additional option of emptying it and storing it away for the winter if that is easier.

The winterising procedure is very important for two reasons. Firstly it helps ensure your pool is in the best condition possible when you come to open it up again in the spring and secondly it helps protect your pool and its fittings from the winter conditions. To winterise your pool simply follow this step-by-step guide;

  1. Backwash the filter and clean the strainer.
  2. Check the pH is between 7.2 and 7.6 and use sodium bisulphate (to lower pH) and sodium carbonate (to increase the pH) to adjust if necessary.
  3. Using Chlorine Shock (calcium hypochlorite), boost the chlorine level to remove any algae that maybe present in the pool. The recommended dosing rate is 1kg per 10,000 gallons of pool water.
  4. To prevent algae from growing add Winterising Algaecide. The recommended dosing rate is 2.5 litres per 10,000 gallons of pool water.
  5. Turn on the circulation to disperse the solution through the water for a period of 24 hours.
  6. Reduce the water level to just below the skimmers. This is to allow for any extra water that enters the pool, ie rainfall, therefore preventing it from overflowing. WARNING: Do not lower the level too much especially if the water table is high as this could cause structural damage to the pool shell.
  7. To protect any exposed metal surfaces, drain the water from the pump(s), filter(s), heater and pipework.
  8. Leave any valves above the water level open. Any below should be closed to protect from frost damage.
  9. Switch off the electricity supply to the system.
  10. Cover the pool with a heavy-duty winter pool cover.
  11. Check the appearance of the water, and the chlorine/pH levels occasionally during the winter.

By following this set of guidelines it should significantly reduce the amount of work you have to do when opening the pool up in the spring. Winterising Kits are available from our website which include all the chemicals you need as well as step-by-step instructions.

Please note the information provided on this page is believed to be correct but is given without warranty and no liability is accepted or implied. Please use this information in conjunction with manufacturers instructions.


Chlorine explained


The most important chemical when maintaining a swimming pool is chlorine. Maintaining the correct level of chlorine in pool water will kill almost all bacteria and viruses introduced into the water by bathers within a few seconds. As a result, the water will remain clear and safe to swim in.

Chlorine is the most commonly used sanitiser in swimming pools and it comes in many forms. The most common types used in pools are:

Liquid Chlorine – Sodium Hypochlorite

Trade Name Chemical Name Strength (Available Chlorine %)
Stabilised Chlorine Granules or “Dichlor” Sodium Dichloro-iso-cyanurate 56%
Stabilised Chlorine Tablets or “Tritabs” Trichioroiso cyanuric acid 90%
Shock Chlorine Granules or “Shock” Calcium Hypochlorite 65 – 70%
Unstabilised Chlorine Tablets Calcium Hypochlorite 65 – 70%
Liquid Chlorine or “Liquid Shock” Sodium Hypochlorite 11 – 15%

Chlorine is also generated electrolytically from salt water. The type of chlorine you buy makes very little difference. The key point which people do not always appreciate is that once the chlorine is dissolved in water it is all the same. Any form of chlorine, when added to water produces hypochlorous acid (HOCl) and it is the hypochlorous acid that kills bacteria, viruses algae in the pool water. This can be referred to as ‘Free Chlorine’.

Contaminants in the water can use up the free chlorine.

Free chlorine is used up by:

  • Pollution from sweat and all the other dirt that washes off bathers – this forms chloramines which cause irritation to bathers and the classic chlorine smell onindoor pools.
  • Contaminants getting into the pool such as leaves, dust, dirt and even rainwater.
  • Sunlight – this breaks up free chlorine in the water in outdoor pools.

So the more people that swim in your pool and the sunnier the weather, the more chlorine your pool requires.

Recommended free chlorine levels

Public indoor pools in the UK aim to keep their free chlorine level between 1.0 and 1.5, however they have sophisticated chemical dosing and control systems. It is normal to keep the free chlorine level in domestic pools (especially outdoor pools) a little higher to give a better margin of safety.  Please see the table below for recommended chlorine levels for pools and spas.

Public Pools with chemical controllers 1.0 – 1.5 ppm
Indoor domestic pools 1.0 – 2.0 ppm
Outdoor pools 2.0 – 3.0 ppm
Hot Tubs and Spas 2.0 – 3.0 ppm


Testing for chlorine

Chlorine is the most important chemical to control and is at least 10 times more important than any other test. Therefore it is vital that you test your chlorine level on a regular basis. You can test for chlorine using DPD1 tablets and a chlorine colour comparator or chlorine test strips.

Chlorine Comparator

Testing hints

  • When the chlorine level is very high it can bleach the DPD1 tablet causing you to get a false low reading (see our troubleshooting blog for more information). Usually the water near the tablet goes pink for a short time and then the colour disappears leaving a slight yellow tinge to the sample.
  • This applies to tests on photometers as well as comparators.
  • If you suspect the chlorine level is high carry out a test using a mixture of 1 part pool water and 4 parts tap (or distilled if you have it) water. Multiply the answer by 5.

How do we increase chlorine level in a domestic pool?

To increase free chlorine quickly

Add stabilised chlorine granules or calcium hypochlorite granules (shock).


  • Add the granules to a bucket of pool water and then add the solution to a deck level drain, strainer basket or direct to the pool.
  • Look at our dosing calculator to work out how much to add to your pool.
  • Be patient – always give it time to mix fully.

To increase free chlorine slowly

To increase chlorine slowly put stabilised chlorine tablets (normally one or two 200gram tablets in a 10,000 gallon pool) in the skimmer or a floating dispenser.

What is the maximum level of chlorine it is safe to swim in?

The UK Pool Water Treatment Group PWTAG – Swimming Pool Water Treatment and Quality Standards – recommends:

  • The pool dosing should be turned off if the free chlorine level reaches 5 ppm.
  • The pool should be cleared of bathers if the free chlorine level reaches 10 ppm.

How do we decrease chlorine level?

By adding sodium thiosulphate solution. This must be done extremely carefully and will be dealt with in a further blog

Introduction to Bromine


Bromine is the most widely used sanitiser for domestic hot tubs and spas. Bromine destroys bacteria, algae, and water-born diseases in much the same way that chlorine sanitises water, however there are a number of key differences between the two.

• Bromine does not form the corrosive by products which cause corrosion of metals in indoor pools

  • Bromine is less volatile than chlorine so it does not evaporate as quickly at the high temperatures which spas operate.
  • Bromine does not form smelly by products, so indoor pools using bromine do not have that typical swimming pool smell. When you have used a bromine hot tub your skin will not smell of chlorine.
  • Bromine does not form the corrosive by products which cause corrosion of metals (including stainless steel) in indoor pools.
  • Bromine does not lose its ability to kill bacteria and viruses at pH values up to 8.2. This is useful on hot tubs in hard water areas where it is difficult to keep the pH down.
  • Bromine cannot be stabilised against breakdown by sunlight, so it is not often used in outdoor pools.
  • Bromine is two and a quarter times as heavy a molecule as chlorine so it is used at a concentration 2.25 times as high as chlorine.

Bromine is normally supplied as white tablets, bromo-chloro-dimethyl hydantoin (BCDMH). In hot tubs these are usually placed in a floating dispenser, which is adjusted to give a bromine level between 3 and 6 ppm. Bromine tablets are very slow dissolving making them very useful in domestic hot tubs. On a hot tub that is used by two people twice a week two tablets in the floating dispenser will keep the water clear and fresh. One extra tablet will need to be added every 7 to 10 days.

Indoor swimming pools usually use an erosion feeder known as a brominator to dose bromine tablets. Because bromine is so slow dissolving it is essential that brominators on busy pools and spas are large enough to meet the bather load. Indoor pools are recommended to maintain a bromine level between 2 & 4ppm.

Bromine is tested using exactly the same test as chlorine using DPD 1 tablets. This is read against color standards in a visual comparator. Since Free Bromine and Combined Bromine are similar in sanitising strength, they both react with a DPD 1 tablet. This reading represents the level of Total Bromine. If a bromine comparator is not available, operators can use a chlorine comparator and multiply the reading by 2.25 to calculate ppm bromine. If you don’t have a comparator bromine can also be tested using bromine test strips.

As mentioned before bromine does not lose its ability to kill micro organisms as pH rises. This means that hot tubs and pools using bromine can use a pH up to 8.2. This is particularly useful on hot tubs and leisure pools where the pH tends to rise if an adequate alkalinity level is maintained.

Preventative action after summer rain storms


After a good start to June the majority of the UK have been treated to heavy storms over the past week. This may have resulted in you forgetting about your pool, however this is often a mistake as pools can go off when there is rain following good weather. The rain washes dust and algae spores into the pool and the dirt uses up the chlorine, allowing  the algae to thrive in the very warm water. The water goes green and it then requires a lot of shocking with chorine and dosing of algaecide to get the pool back to its previous pristine condition. It is therefore important to take pre-emptive action to prevent this by following these simple steps;

  • Give the pool a shock dose of 500grams  of calcium hypochlorite granules (shock chlorine) for every 10,000 gallons of pool water. You can use stabilised chlorine granules if you have an above ground pool.
  • Add an algaecide to your pool.
  • If you use stabilised chlorine tablets or multifunctional tablets put an extra tablet in the skimmer and run the pool at the high end of the chlorine range.

Even though you are unlikely to use your pool during thundery weather, using some extra chemicals now will save you a lot of effort and chemicals in the medium term and it will allow you to use your pool when the good weather returns.

Video – How to clear a green pool


Want to know how to clear a green pool? Check this video out to see how to get your pool from green to clean!



What is a comparator test?


Comparator Test Kit with Disc

If you are looking for an alternative to test strips when it comes to analysing your pool or spa water then you might consider using a comparator test kit. A comparator is an item of testing equipment used to visually test pool water for parameters such as free chlorine, total chlorine, total bromine and pH. Usually they come in the form of a comparator disc and unit but there are more basic kits on the market that don’t have a disc and just have a colour scale on the side of the test tube.

To conduct a comparator test the first thing you need to do is rinse the test tubes out with tap water. This removes any traces of previous tests that could affect the result. You then need to take two 10ml samples of water. One will be used as a blank in the comparator unit and the other will be used to perform the test. You then need to add the reagent to the sample and crush it using a crushing rod. When doing this you will see that the sample changes colour (if you are doing a chlorine test then it will hopefully turn a shade of pink). Once the tablet has fully dissolved you need to hold the unit up to the light and look through the view finder on the comparator unit and turn the comparator disc until the two colours match. When you have achieved the closest match, read the relevant number off the disc to determine the level in the water.


Basic Comparator Test Kit

There are normally three tests you can do using a comparator and these are;

  • Free Chlorine or Bromine using a DPD 1 tablet.
  • Total Chlorine using a DPD 3 tablet.
  • pH using a phenol red tablet.

If you are using a basic comparator test kit then the process is similar but you don’t use a blank sample. Instead you fill the relevant compartment up with water, add the reagent and then compare the colour of the sample to the colour chart on the face of the compartment.


Opening your pool for the Spring?


As we move into March most pool owners across the country will start thinking about pulling back the pool cover and getting the pool ready for the spring. If you winterised the pool at the beginning of last winter then opening the pool should be a relatively straightforward process, but nevertheless here is a simple step-by-step guide on how to get your pool ready.

Preparing the Pool

  1. Remove any leaves and debris from the winter cover. Take the winter cover off, clean it and store it away.
  2. Connect the pool pump and filter and reconnect all hoses and electrical connections.
  3. Remove leaves and debris from skimmers, filters, pumps and drains.
  4. Remove any debris and leaves that are in the pool using a leaf skimmer.
  5. Fill the water up to the proper water level. The water should be halfway up the skimmer opening.
  6. Turn on the filter pump and make sure all skimmers, bottom drains and filters are functioning properly.

Adjusting the pH and Alkalinity Levels

  1. After the pool filter has been running for 3-4 hours, test the pH and alkalinity levels and compare with the table below.
  2. To reduce the pH level use pH Reducer. Dissolve a small amount (450g per 10,000 gallons) in a bucket of pool water and distribute it around the pool with the filter operating. Retest after 4 hours.
  3. To increase the pH level first adjust the Alkalinity to between 60-100ppm. To increase the Alkalinity level by 12ppm add 1kg Alkalinity Builder per 10,000 gallons directly to the pool with the filter operating. Retest the Alkalinity level and the pH level after 4 hours.
  4. If the pH level is still too low dissolve a small amount (450g per 10,000 gallons) of pH Increaser in a bucket of pool water and distribute it around the pool with the filter operating. Retest after 4 hours. Repeat this dosage if necessary.
Low OK Ideal OK High
pH Less than 7.0 7.0-7.2 7.2-7.6 7.6-7.8 Over 7.8
Total Alkalinity Less than 40ppm 40-60ppm 60-100ppm 100-120ppm Over 120ppm

Shock Treat the Pool

  1. Brush the pool walls and steps and vacuum up any debris. This will expose the algae to the chlorine.
  2. Dissolve 500 grams per 10,000 gallons of Chlorine Shock in a bucket of pool water and distribute around the pool. DO NOT ADD TO THE SKIMMER BASKET IF IT HAS HAD STABILISED CHLORINE IN IT AS IT MAY EXPLODE IF MIXED!!!
  3. Depending on how green the pool is it might require several shock treatments. If your pool is very green you can double the initial shock dose. Following the shock dose, backwash the filter to remove any debris.

Stabilising the Chlorine Level

  1. After a few days your chlorine level should have returned to normal (see table). Once it has you can now add a 200g Multifunctional tablet or 200g Stabilised Chlorine tablet to your skimmer basket or dispenser.
  2. To improve the clarity of the water add Sparkle Water Clarifier to a bucket of pool water and distribute around the pool.
  3. To prevent algae re-occurring add Polyquat Algaecide to a bucket of pool water and distribute around the pool.
Low OK Ideal OK High
Chlorine Less than 0.5ppm 0.5-1.0ppm 1.0-3.0ppm 3.0-5.0ppm Over 5.0ppm

Daily Maintenance

  • Monitor Chlorine, pH and Alkalinity levels using AquaChek Free Chlorine Test Strips.

Weekly Maintenance

  • Clean the skimmer baskets and brush the walls, steps and bottom of the pool. Vacuum if necessary.
  • Clean the strainer basket before the pump. When this is complete backwash the filter.
  • Shock dose the pool. Dissolve the required amount (see instructions) of Chlorine Shock in a bucket of pool water and distribute around the pool. DO NOT ADD TO THE SKIMMER BASKET IF IT HAS HAD STABILISED CHLORINE IN IT AS IT MAY EXPLODE IF MIXED!!!
  • Add Sparkle Water Clarifier to a bucket of pool water and distribute around the pool.
  • Add Polyquat Algaecide to a bucket of pool water and distribute around the pool.

Dosing Information – Health and Safety

  • Always wash out the container you mix chemicals in thoroughly with pool water before and after use.
  • Do not ever mix chemicals together
  • Always add chemicals to water not water to chemicals.

Calcium Hypochlorite Granules (Chlorine Shock)


Calcium Hypochlorite (Chlorine Shock) is an unstabilised form of granular chlorine that is most commonly used as a shock treatment in swimming pools. It has 70% available chlorine which makes it stronger than other shock treatment chemicals such as Sodium Hypochlorite (14-15%) and Stabilised Chlorine Granules (56%). Another major advantage of using calcium hypochlorite as a shock treatment is that it doesn’t contain any stabiliser. There can be occasions where you need to add a large shock dose in order to kill algae. If  you used a stabilised chlorine like the aforementioned Stabilised Chlorine Granules, then it would still kill the algae but it would also increase the stabiliser level in the water. Over time the stabiliser level will build up  and eventually it will get too high and you will have to reduce it by draining a proportion of pool water and replacing with fresh water.

Calcium hypochlorite granules should be dissolved in a bucket of warm water and distributed around the pool or poured into the skimmer basket. HOWEVER please make sure your skimmer basket is free from all other types of chlorine before pouring it in. The reason for this is if you mix unstabilised chlorine and stabilised chlorine together in their raw form they will react violently. Once they are added to the pool separately they are fine. People often use a trichlor tablet as their sanitiser and calcium hypochlorite granules as their shock treatment. If this is the case put the trichlor tablet in the skimmer basket and when it is time for a shock treatment, dissolve the calcium hypochlorite granules in a bucket of warm water and pour the solution around the pool. This will ensure you keep the two types of chlorine separate from one another.

It is not recommended to use calcium hypochlorite granules in a spa as it is undesirable to have large amounts of calcium build up in a spa. It will cause scaling of the heater and white deposits on the sides therefore we recommend using Oxy Shock instead.

How does your water supply affect your Hot Tub maintenance?


Did you know that your water supply plays a significant role in how you go about maintaining your hot tub water? The water supply in our homes is usually referred to as hard or soft and it is important to know your water supply as it will affect some aspects of your hot tub maintenance.

A quick way to determine whether you have hard or soft water is to take a quick look in your kettle. If you can see signs of scale then your water supply will be classed as hard. If you can’t then it is likely to be soft. If you are unsure you are best to check with your local water board.

If you live in a hard water area then you need to be aware of two main issues; scale and a high pH level. I mentioned earlier about the build up of scale in your kettle. The chances are it will be a similar picture on the internal pipe work and heater in your hot tub. If left untreated then this could eventually cause significant damage, especially to the heater. With this in mind it is important that you use a scale inhibitor such as our Stain and Scale Inhibitor.

The main chemical that makes water hard is calcium bicarbonate. The calcium part is the hardness whilst the bicarbonate is the alkalinity. As a result the second problem you will probably encounter is a high pH level and high alkalinity level. Both of these issues can be rectified by adding a pH Reducer, which is usually sodium bisulphate. If you do live in a hard water area it may be prudent to run your hot tub on bromine as you can maintain a pH level between 7.2 and 8.2. If you run on chlorine then you must maintain a pH level between 7.2-7.6.

If you live in a soft water area then the opposite will happen as your water supply will have a low pH level and low alkalinity level. First of all you need to get your alkalinity level right and you can do this by adding an alkalinity builder, which is sodium bicarbonate. Once you have added this you will find that it slowly also brings your pH level into the desired range.