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This group of chemicals is the most important in treating your pool. Their job is to quickly kill any bacteria or viruses that get into the water so that they cannot multiply and cause infections in swimmers. Consequently they play a major role in keeping the pool water clean and clear.

There are two main sources of sanitisers:

There are two types of chlorine available to use as sanitisers. These are stabilised and unstabilised chlorine.

Stabilised Chlorine
Stabilised chlorine is predominantly used in outdoor pools. In order to stop chlorine being broken down by sunlight it is combined with a stabiliser called Cyanuric acid. Cyanuric acid can be added separately or more commonly it is supplied as stabilised chlorine granules or tablets.

Stabilised Chlorine Granules are a white granular solid and are added to the pool water via manually dosing your pool as and when needed. They have a pH close to neutral (7) so they will have little effect on the pH of the pool water. They are added to the pool water by dissolving the required amount in a bucket of warm water and distributing it around the pool.

To increase the free chlorine by 1 ppm, add 100 grams per 10,000 gallons.

Stabilised Chlorine Tablets are provided in a 20 gram and a 200 gram tablet. The 20 gram tablets are designed for smaller above ground pools whilst the 200 gram tablets are designed for larger in-ground pools with a larger volume of water. They are added to the water via a floating dispenser or the skimmer basket. As they are slow dissolving they can be left in the pool, providing a constant chlorine dose at all times. This reduces the amount of work you have to do to maintain an adequate chlorine level. As they are slightly acidic it is important to keep an eye on your alkalinity and pH levels.

Multifunctional Tablets are a type of stabilised chlorine tablets that work in exactly the same way. However these tablets have an algaecide and a flocculant built in allowing you to reduce the chances of algae and cloudiness occurring whilst dosing chlorine.

For 20 gram tablets fill the floating dispenser up and adjust the collar accordingly. The tablets will take 5-7 days to dissolve.

For 200 gram tablets place one tablet per 11,000 gallons in the skimmer basket. It will take 3-5 days to dissolve. Alternatively place one tablet per 7,000 gallons in the floating dispenser. It will take 10-14 days to dissolve.

Unstabilised Chlorine
Calcium hypochlorite is supplied as an off white granule or tablet. The granules are dissolved in a bucket of warm water and poured into the skimmer basket. It is important to pour it into your skimmer basket rather than your pool as the granules will not completely dissolve in the bucket. Please make sure your skimmer is free from all other types of chlorine before pouring it into the skimmer. The tablets are placed in the skimmer basket or floating dispenser. As it is an unstabilised form of chlorine it doesn't contain cyanuric acid and is therefore mainly used in indoor pools where the chlorine is not affected by the sunlight. If stabilised chlorine is used for indoor pools it can lead to high levels of cyanuric acid.

75 grams adds approximately 1 part per million of free chlorine to 10,000 gallons of water.


Some pools, both indoor and outdoor, are fitted with circulation feeders called brominators. These use a sanitiser called bromo-chloro-dimethyl hydantoin (BCDMH). Spa pools often have brominators. As a result of a complex series of chemical reactions the active sanitiser becomes hypobromite rather than hypochlorite. BCDMH is supplied in the form of white tablets which are loaded into the brominator and the pool water is pumped over them. BCDMH should not be added directly to the pool or through the skimmers.

The level of active bromine in the pool is tested with a bromine test kit and the brominator is adjusted to give a bromine level between 2 and 4 ppm. The pH level can be allowed to rise to between 7.8 and 8.0 so very little pH minus is required. As the chlorine and bromine in BCDMH are used up the amount of the DMH part of the chemical in the pool increases. Excessive DMH affects the disinfection efficiency so the level must not be allowed to get too high. The concentration should not exceed 200 ppm. Unfortunately, there is no simple test for DMH but regular filter backwashing and an adequate dilution policy should keep the level below 200 ppm.

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