Water Treatment

Tel: 01527 861321

Chlorine & Stabiliser

Note: Dosing rates may vary depending on the manufacturer/brand, always read the label before applying chemicals to swimming pool water.


There are three main types of chlorine;

Total Chlorine – the total amount of chlorine in the swimming pool

Free Chlorine – The chlorine that is still available to be used in the water

Combined Chlorine – the chlorine that has been used up in the water

If the Total Chlorine in your pool is higher than the Free Chlorine reading, then the difference between the two represents the level of Combined Chlorine in the water. If the readings are the same, then no Combined Chlorine is present. The Total Chlorine level cannot be less than the Free Chlorine level.

Once you know how much Combined Chlorine is in the water, you should add about 10 times that amount of Free Chlorine to get rid of it. This is one reason why we shock treat a swimming pool.


Trichloroisocyanuric Tablets -  Chlorine Tablets – They are designed to be placed in the skimmer or a floating dispenser and release the chlorine slowly into the water, these tablets contain stabiliser so should only be used to maintain the chlorine level in the swimming pool and not for shock treatment.

Trichloroisocyanuric Multifunction Tablets – Multifunction Chlorine Tablets – These tablets are similar to normal chlorine tablets but contain extras, like algaecide and flocculant, once again they contain stabiliser so should only be used to maintain the chlorine level in the swimming pool and not be used for shock treatment


Sodium Hypochlorite – Is liquid chlorine, it has 10-15% available chlorine, although it can be added directly to the swimming pool water care should be taken because it can bleach the liner and your clothes. Sodium Hypochlorite is un-stabilised so can be used as shock treatment for swimming pools.

Calcium Hypochlorite – is granular chlorine, it has up to 70% available chlorine, depending on the brand, it should be pre-dissolved in water prior to adding to the swimming pool water. Calcium Hypochlorite is un-stabilised so once again can be used as a shock treatment for swimming pools, but, unlike Sodium Hypochlorite, Calcium Hypochlorite contains calcium, depending on your area, the water may already have a high calcium content in which case Sodium Hypochlorite may be better for you to use.


Shock Chlorine Treatment - A swimming pool shock treatment is recommended  to kill algae and bacteria, it is also used to get rid of the used up chlorine(combined chlorine) still left in the swimming pool. Before shock treating your swimming pool make sure the PH and alkalinity levels are correct, if not adjust them first. Pre-dissolve the shock granules in a bucket of pool water. The chart below shows the dosing rates

Water Volume

In Litres

Water Volume

in Gallons

To Increase Chlorine

Level by 1ppm

To Increaser Chlorine

Level by 5ppm

To Increase Chlorine

Level by 10ppm

4,545 1,000 7g 35g 70g
11,360 2,500 17g 88g 175g
22,725 5,000 35g 175g 350g
68,175 15,000 105g 525g 1050g

See the shock treatment in relation to stabiliser chart below to find the recommended level needed to shock treat your swimming pool. Check the free chlorine level of your swimming pool by using a good quality test kit or test strips then using the chart above, increase your free chlorine content to the recommended level

For Example

If your swimming pool is 15,000 gallons and your stabiliser level is 20, and your free chlorine test result is 3ppm then to shock treat you will need 735g of shock treatment to bring the free chlorine level up by 7ppm to 10ppm.

Note: Dosing rates may vary depending on the manufacturer/brand, always read the label before applying chemicals to swimming pool water. 

Cyanuric Acid – Stabiliser – Some forms of chlorine contain stabiliser which protects the chlorine from ultraviolet rays, but just the right amount of stabiliser is very important. One way to understand the job of the stabiliser is to think of it as a bubble around the chlorine, if the bubble is too thin (a low level of stabiliser) then the chlorine will be used too quickly, but if the bubble is too thick (a high level of stabiliser) then the chlorine won’t be used up effectively and you will find yourself having to add more chlorine to compensate. To raise the stabiliser cyanuric acid is added in the form of granules but, the only way to lower the stabiliser level water should be drained from the swimming pool. See the chart below for the relationship between Chlorine and Stabiliser.


Cyanuric Acid

Minimum Free


Target Free


Shock Treatment

Free Chlorine

20 2 3 10
30 2 4 12
40 3 5 16
50 4 6 20
60 5 7 24
70 5 8 28
80 6 9 31
90 7 10 35
100 8 11 39


For example;

If the stabiliser level is 30 then the minimum free chlorine level should be 2ppm, but the recommended level should be 4ppm, and if you shock dose the swimming pool a free chlorine level of 12ppm should be reached.

Note: Dosing rates may vary depending on the manufacturer/brand, always read the label before applying chemicals to swimming pool water. 


Bromine comes in granular or tablet form and is an alternative to chlorine. It is more stable than chlorine in warmer water temperatures.  

Bromine combines with bacteria in pool water to neutralize it in the same way that chlorine does, however a good portion of the bromine stays active even after combining with the bacteria. 

The shock treatment will burn off the bacteria leaving the bromine in the pool water to sanitize the pool again. 

Because of this, the level of bromine in the swimming pool water is less than that of the chlorine level. People with sensitive skin will use bromine over chlorine.

Active Oxygen

This is another alternative to chlorine, known as Potassium Monopersulfate or MPS.

When chlorine is used to oxidise pool water it forms chloramines which produces an odour, but because active oxygen doesn’t contain chlorine, when it is used to oxidise pool water it won’t leave any smell.

It’s not as sensitive to sunlight as chlorine and bathers can re-enter the pool a short time after the water has been shock treated with active oxygen unlike chlorine.

It also works well as a shock treatment alongside a weekly chlorine treatment. While active oxygen is a strong oxidant, it is not a sanitizer, and therefore provides no protection against bacteria and viruses. It is also more expensive than chlorine.

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Walkmills Farm, A38 Wychbold, Nr Droitwich, (100 yds off junction 5, M5 Motorway), Worcestershire, WR9 0DH, United Kingdom.

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