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Useful terms for swimming pool and hot tub/spa owners 




Algae are microscopic plants which enter your pool either from the air, or are washed in or carried in on pool covers or even bathers. Their most common colour is green but yellow black strains occur too. They flourish in pool water conditions in bright sunlight. A particularly suitable condition to propagate algae is if fertiliser from an adjacent lawn has been washed into the pool. Algae cause the pool water to become opaque, which is dangerous if a bather has got into difficulty unseen. Algae can cling in an unsightly manner on the pool walls and other surfaces. Algae problems can be minimised by keeping the sanitiser in the pool at the correct level.


An algicide is a chemical or other treatment which either kills or inhibits the multiplication of algae. The normal sanitisers such as Chlorine and Bromine will keep algae in check but if they multiply in an uncontrolled manner an special algicide treatment will be required.

Aluminium Sulphate (commonly known as Alum)

A chemical added to water which causes tiny particles to agglomerate together so that they may be more easily filtered.
Go to Instructions for using Aluminium Sulphate.


An additive often based on silicones, used principally for spa baths to counteract foaming.

Available Chlorine

The amount of chlorine in water available to sanitise. Also called free chlorine or residual chlorine. The activity of a particular sanitiser may be stated eg. Dichlor is typically 56% available chlorine. The lowest level of free chlorine likely to give adequate sanitising, if all the other parameters are satisfactory and the bather load is low is 1 part per million. More likely 2 to 3 PPM will be used. Shock Chlorination, during which time bathers should not be in the water is 5 to 10 parts per million.




A process whereby the circulation through a filter is reversed to waste, to flush away contaminants. It should be done weekly in commercial pools or regularly but less frequently in domestic pools depending upon bather load. It should also be carried out if the pressure loss across the filter is greater than that recommended by the filter manufacturer. With sand filters, the backwash pressure should be great enough to disturb the sand ("fluidised bed"). Generally this carried out with no bathers in the pool. With sand filters, it is good practice after backwashing to run in the normal flow direction to waste to allow the sand bed to settle. The discharge from commercial pools is classified as trade effluent and a discharge consent must be obtained from the appropriate discharge authority.


See Pathogenic Organisms

Bather load

The number of persons using a pool in 24 hours. These are the main source of contamination and at times of high bather load it is important to ensure the water conditions, particular sanitiser and pH are correct.


A bromine sanitiser. The initials refer to "bromochlorodimethylhydantoin. When BCDMH dissolves in water it produces a solution of Hypobromous Acid and Hypochlorous Acid. The latter breaks down bromamines to regenerate the bromine sanitiser which has been consumed by forming these compounds. Popular in spa baths.

Bleaching Out

At above 10 chlorine in a pool it is possible that the colour developed by the use of DPD tablets will be bleached. This can result in the tester believing there is no chlorine in the water and adding more when there is too much present in the first place. When testing you may get an initial flash of pink colour which rapidly disappears. If you suspect that the colour is bleaching, try diluting the pool water sample 1:1 with unchlorinated water, and then if necessary repeat 1:2 and 1:3. Retest each time and if by diluting the pool sample you then get a colour change, you will know that the pool is over chlorinated and that the chlorine is bleaching the indicator.

Breakpoint Chlorination

Chloramines are undesirable in pool water. Monochloramine is not a great problem as it does not irritate and has some sanitising value. Dichloramine however irritates the eyes and nose and is the source of complaints attributed to chlorine generally. When measuring total chlorine the value attributable to Monochloramine + Dichloramine + Free Chlorine will rise as more chlorine donor is added. When sufficient is present the chloramines will decompose and the combined chlorine level will fall whilst the free chlorine continues to rise. This is known as the breakpoint and is an indication that the chloramines have been oxidised.


Bromine, like Chlorine and Iodine and Fluorine is an element of the halogen group. All these halogens have sanitising properties. Bromine and Chlorine are used in pool waters. Iodine would work but stains heavily. Fluorine would etch polished ceramic surfaces.

 BPR (Biocide Product Regulations)

Legislations impacting on biocides (Copper Algaecides, Chlorine, Bromine, Hydrogen Peroxide, Potassium Monopersulphate – Non Chlorine Shock) sold within the European Union (EU). Only biocide products registered to the BPR can be sold within the EU from 1st September 2015.


A chemical such as Sodium Bicarbonate, which when dissolved in water, reduces the tendency for pH changes to fluctuate wildly.



Calcium Hypochlorite

This is a form of Chlorine sanitiser in powder or granular form. It contains 65% available Chlorine and is unstabilised. Because of this it is suitable for shock chlorination where you need to get a lot of Chlorine into the water quickly. Also, if the use of stabilised Chlorine has built up too much stabiliser in the water, Calcium Hypochlorite can be used as an alternative. Because it contains Calcium, it will raise the Calcium hardness of the water. This can be a good thing if you are trying to raise the hardness in a soft water area or it can be undesirable if you have too much Calcium and it is causing scaling problems. As it is alkaline, Calcium Hypochlorite will raise the pH of the water.

Calcium Hardness

Most of the hardness of your water supply is dissolved Calcium salts due to the water having arisen in Chalk or Limestone areas. Magnesium salts also tend to be grouped with Calcium in this description. Some hardness is good for your pool otherwise Calcium from sources such as tile grout will be dissolved. Conversely, too much causes scaling problems.

Calcium hungry

If water is very soft i.e. it contains little dissolved Calcium from a hard water source, it will then dissolve Calcium from other sources such as concrete and tile grout. The use of Calcium Chloride (Water Hardness Increase) will counteract this, as may the use of Calcium Hypochlorite as a sanitiser.

Cal hypo

A shortened version of the name of Calcium Hypochlorite used by some in the pool trade.


See Sequestrant


Combined chlorine products are formed by a reaction between nitrogen compounds often derived from bathers (think about it!). These chloramines are responsible for unpleasant odours and eye irritation. They can be broken down by raising the free chlorine level.


By products in the water resulting from the use of any chlorine product. Principal component of Total Dissolved Solids


The most common sanitiser found in pools. Like Bromine, it is a member of the halogen family. It dissolves in water to give hypochlorous acid which sanitises the water. Chlorine is provided by various chlorine donors each with advantages and disadvantages. These are:-


Advantages and Disadvantages

Chlorine Donor

Available Chlorine %

Physical Form


Sodium Hypochlorite

10 to 14


Cheapest chlorine donor. Poor shelf life in hot weather. Corrosive. Raises pH. Popular in commercial pools with trained staff.

Sodium Dichloroisocyanaurate



Stabilised chlorine. Good for general sanitising. Relatively low hazard and easy to use makes it popular for domestic pools.

Calcium Hypochlorite


Powder or Granules

Unstabilised. Good for shock chlorination. Good for over-stabilised conditions, otherwise a separate stabiliser is required. Raises pH and Calcium hardness of the water. Popular in municipal pools.

Trichloroisocyanauric Acid



Easy to use. Add to skimmer for slow dissolution. Not suitable for shock treatment.

Elemental Chlorine



Suitable only for large professionally run pools with specialised equipment.


Chlorine products can beneficially be added at night to your water, when there is no ultraviolet light to degrade them. This way the chlorine has chance to work on the contaminants in your pool overnight.
See also Bleaching Out
See also Breakpoint Chlorination

Chlorine Demand

The chlorine required to destroy the organic contaminants of the water.

Chlorine Donor

A chemical which when dissolved in water provides a solution of Hypochlorous Acid. See Chlorine

Chlorine Lock

A situation in which the cyanuric acid stabiliser has built up through continuous use of stabilised sanitisers. This slows down the rate at which hypochlorous acid forms in solution. There are two possible remedies. Either introduce fresh water into the pool or use unstabilised chlorine such as Sodium or Calcium Hypochlorite.

Chlorine Residual

Any excess of Chlorine after the chlorine demand has been met.

Chlorine Smell

The smell and irritation associated with chlorine is probably unfairly attributed to the chlorine itself. More likely a reaction has taken place with the amine soiling in the pool and chloramines have been formed, in particularly dichloramine and worse trichloramine. It is these compounds which cause irritation, not just 2 - 3 ppm of free chlorine. Regard this as a sign that there is something wrong with the pool water treatment and probably it is necessary to add more unstabilised chlorine to "shock" the pool water.

Combined Chlorine

A reaction product between hypochlorous acid and nitrogen compounds such as amines from bather pollution.


A substance used to promote water clarity by improving the filtration characteristics of the haze particles. Various terms are use to describe clarification such as flocculate, coagulate and agglomerate. Whatever the term, they work as follows:

Turbidity of pool water is sometimes caused by tiny particles which are negatively charged. As each particle has the same charge, they repel each other and cannot coagulate together to allow removal by the filtration system. The addition of a cationic (positively charged) polymer to the water attracts the negatively charged particles to it and allows coagulation.


A test kit for measuring some of the parameters of pool water such as pH and Chlorine. A reagent tablet, liquid or powder is added to a sample of the pool water. A colour develops in the sample which is then compared to a reference colour chart. From the best match of the colours, the result can be directly read off. See also Photometer


An alternative name for stabiliser. See Cyanuric Acid


It is by measuring the electrical conductivity of the pool water that a value for total dissolved solids can be found. The conversion between Conductivity and Total Dissolved Solids is Conductivity in microsemens/cm X 0.7 = TDS in mg/litre


A product caused by the catabolism of creatine, a substance present in urine.

Cyanuric Acid

A stabiliser either combined with the chlorine donor (Dichlor and Trichlor) or added separately to water treated with Calcium or Sodium Hypochlorite. It helps protect against chlorine loss in strong sunlight. The Cyanuric Acid range is recommended to be 50 to 100 mg/litre. Excess cyanuric acid and certainly over 200mg/litre should be avoided as it causes chlorine lock.




Sometimes, either as a result of accidental over chlorination, it is necessary to lower the chlorine level of pool water. This is achieved by treatment with either Sodium Thiosulphate or Hydrogen Peroxide. As an example for each 1 p.p.m. (mg.l)chlorine in 45,000 litres there will be 450gm chlorine. For each gram of chlorine add 2gm Sodium Thiosulphate or 1.5gm 30% Hydrogen Peroxide.


See antifoam


A commonly used name for Sodium Dichloroisocyanurate Dihydrate. This is a very common granular form of stabilised chlorine donor giving about 55% available chlorine. In solution it disassociates into Hypochlorous Acid, which sanitises and cyanuric acid which stabilises chlorine.


A chemical or other treatment used to destroy pathogenic organisms.


The indicator usually used in tablet form which measures chlorine and bromine levels in pool and spa water. DPD is an abbreviation for Diethyl-p-Phenylene Diamine.

DPD1 tablets are used for measuring free chlorine. DPD3 tablets are for total chlorine.
See also Bleaching Out.

Dry Acid

A powdered chemical such as Sodium Bisulphate or Sulphamic Acid, which is acidic in nature when dissolved in water. Used to lower pH these are sold as pH Decrease or pH Minus.

Dry Chlorine

A term sometimes used to describe Calcium Hypochlorite or Dichlor
See how to use these materials.




Trade Mark of the world's largest range of chlorinated isocyanuric products.




An infill material between tiles. This can be damaged by too soft water (Remedy - Add Calcium Hardness Increase) or high Sulphate content in the water. (Remedy - Use a pH Decrease based on Sulphamic Acid or Hydrochloric Acid).




Trade Mark of a worldwide supplier of water treatment chemicals, most notably Calcium Hypochlorite. We sell Calcium Hypochlorite sourced from a variety of manufacturers under its chemical name. HtH stands for High Test Hypochlorite.

Hard Water

When rainwater passes through rocks such as chalk and limestone, a certain quantity of mineral salts such as Calcium and Magnesium Carbonate dissolve into it. These hardnesses salts are then carried into the mains water supply. Most commonly these are expressed as p.p.m. of Calcium Carbonate. Anything in excess of 250 p.p.m. is considered hard. If you live in the South of England on Chalk or in the Pennines on Limestone then your water will be very hard, in excess of 500 p.p.m. If you have hard water you will have noted "furring" of kettle elements and may have fitted a water softener to avoid problems with boilers and dishwashers. In pool and spa water, some hardness is desirable as it prevents corrosion of pool surfaces and stops tiling grout dissolving into the water. If on the other hand it is too hard, then scaling will result. This is unsightly at best and can cause damage to boilers at worst.


The group of elements comprising chlorine, bromine, iodine and fluorine. Of these, chlorine and bromine are used to sanitise pool and spa waters.

Hypobromous Acid

When BCDMH is used to sanitise a pool or spa bath water, it provides bromine in the form of hypobromous acid. Hypobromous acid combines with nitrogen compounds from bathers to produces bromamines. Unlike chloramines, there is no irritating odour associated with bromamines. The hypobromous acid can be regenerated by shock treatment with OXYGEN SHOCK.

Hypochlorite Ion

If the water pH is too high, hypochlorous acid disassociates into a hydrogen ion (H+) and a hypochlorite ion (OCl-). If the pH is too low, chlorine is evolved which will result in irritation to bathers. Due to its negative charge, hypochlorite ions are repelled from the cells walls of the micro-organism they are trying to penetrate. In consequence hypochlorite ions are very poor sanitisers by comparison to hypochlorous acid.

Hypochlorous Acid

When a chlorine donor dissolves in water, hypochlorous acid is formed and it is this compound which sanitises the pool water.




A well-known British brand of Copper Long Life Algaecide.



Langelier Saturation Index

This is a useful formula to use to determine the scale forming or corrosive nature of water.

Saturation Index = pH + CF + AF + TF - 12.1

CF is the Calcium Hardness factor from the table below AF is the Total Alkalinity factor from the table below TF is the Temperature in Degrees Centigrade factor

If the result of this calculation is zero then the water is balanced. If the answer is less than 0 then the water will tend to be corrosive. If it is greater than 0, then it is said to be scale forming. An acceptable range is -0.5 to +0.5. If the result is outside this range, adjustments should be made to the hardness, alkalinity and pH to bring it within the optimum range.


Calcium Hardness

Total Alkalinity






Degrees C








































































Muriatic Acid

A rather old fashioned alternative name for Hydrochloric Acid. This is sometimes used in professionally managed pools to lower pH. Its advantage is that, being liquid, it can be automatically dosed. Domestic and smaller commercial pools usually use a powder pH decrease.



Non Chlorine Shock

Active oxygen addition in the form of Potassium Peroxydisulphate can be used as an alternative to chlorine shock treatment. This is particularly effective for use with BCDMH sanitisers for spa baths as it is odourless and generates free hypobromous acid.




An abbreviation for ortho- Tolodine. At one time, the was the indicator used for the chlorine test. As this is now believed to be a Harmful substance, it has now largely been replaced by DPD.


A gas used as a bactericide in some commercial pools but now being used in some spa baths.



Parts Per Million (often abreviated to p.p.m.)

When a small measure of a substance is present as a mixture or solution in another it is frequently described in parts per million. For example 1 p.p.m available Chlorine in water. This can also be given as 1 gram per litre.

Pathogenic Organisms

Microscopic organisms such as bacteria, virus, protozoa and moulds. These enter the pool as contamination from bathers, birds or from the air or from dust or rain.

Those of concern to pool and spa are:

*  Cryptosporidium parvum

A protozoa causing diarrhoea. Unfortunately this is resistant to chlorine sanitisers but should be removed by efficient filtration.

*  Giardia lamblia

A protozoa similar to cryptosporidium.

*  Pseudomonas aeruginosa

Bacteria causing ear and hair follicle infections.

*  Shigella

Bacteria causing dysentry.

*  Adenoviruses

Viruses causing throat and eye infections.

Organisms which also cause concern are:

*  Escherichia Coli ("E coli")

This bacteria is most often associated with "food poisoning" but can be spread from person to person.

*  Legionella pneumophila

This causes respiratory infections and is spread through water in fine droplets (aerosols). It is frequently associated with cooling water and air conditioning for this reason. In swimming pools, the conditions to promote legionnaires disease are unlikely to occur. They may occur though in aerated spa baths, fountains, waterfalls and showers.

HSE Information Sheet Legionnaires' Disease: Controlling the risks associated with using spa baths

*  Leptospira

Bacteria from the urine of infected rats. Needless to say you shouldn't have this in your pool!


A logarithmic scale indicating the degree of alkalinity or acidity of a solution. The scale is between 0 and 14. 7 is the neutral point (the measure of pure water). Less than 7 indicates acidity. More than 7 indicates alkalinity. Each unit of 1 indicates a change of ten times. ie. pH 9 is 10x more alkaline than pH8 or pH3 is 10x more acidic than pH4. The term pH is derived from "potenz" and H for Hydrogen, in other words its potential (in German) to yield hydrogen. ions.

pH is most important to the quality of pool and spa water. The ideal point is close to the neutral position of 7 but the following should be considered

If the pH is too low then corrosion of pool surfaces and equipment may occur and it may be uncomfortable for bathers. If it is too high then the production of hypochlorous acid will be adversely affected and so sanitisation will be reduced. If it rises above 8 then Calcium Carbonate may be precipitated and the water may become cloudy which is dangerous, unsightly and scale problems may occur.

What is the ideal pH? The compromise of all the factors is to control the water between pH 7.2 and 7.6

ph Decrease ("pH minus")

An acidic material added to water to lower the pH.

ph Increase ("pH plus")

An alkaline material added to water to raise the pH.

pH measurement

This may be carried out by test strips, test tablets (phenol red) or by using a pH meter. See test kit section


An instrument for testing pool water conditions, which measures the colours in a similar matter to a comparator, but as the results are read electronically rather than visually, then they are more accurate.




A substance or process which kills or inhibits the multiplication of pathogenic organisms such as bacteria and moulds.
An example would be a quaternary ammonium compound or ultraviolet light. This is not quite the same as sterilisation in the sense of clinically clean. Sanitised describes a state where the level of pathogenic organisms present does not present a health risk. See also disinfection.

Sauna Milk

An emulsified aromatic fragrance which is mixed with water and thrown onto the hot rocks in a sauna.


A chemical which holds metals in solution and help prevent scaling. Examples of metals occurring in pools would be the Calcium and Magnesium of hard water salts and Copper from pipework. At the concentrations that it is safe to bathe in, they cannot be expected to dissolve scale that has already formed.

Shock Chlorine

If pool water has problems of an algae build up or chlorine odour it may be necessary to increase the chlorine level from its normal 1 to 3 p.p.m to between 5 an 10 p.p.m. This treatment should be carried out without bathers in the pool and they should not be permitted to return until the chlorine level has dropped to its normal level. This process should kill microorganisms and algae and break down the chloramines which are responsible for the undesirable chlorine odour. The preferred chlorine donors are Sodium Hypochlorite or Calcium Hypochlorite as these are unstabilised and release their Chlorine quickly.

Shock Treatment

This term describes adding additional oxidising agent for the purpose of destroying chloramines and algae. It should be carried out when bathers are not in the pool.

Sanitiser in Use

Shock Treatment

Dichlor (Stabilised Chlorine Granules)

Calcium Hypochlorite

Trichlor (Stabilised Chlorine Tablets)

Calcium Hypochlorite

Calcium Hypochlorite + Stabiliser

Calcium Hypochlorite

Calcium Hypochlorite

Calcium Hypochlorite

Sanitisers with no oxidising properties e.g Baquacil, Revacil, Blue Crystal

11.9% Hydrogen Peroxide

Bromine Tablets

Oxygen Shock


See Shock Chlorine


A device fitted on the water level line used to strain out debris. Some designs lend themselves to allow Triclor tablets to be put in them for gradual dissolution into the pool water

Soft Water

Distilled or demineralised water which has no dissolved metals or Carbon Dioxide is the ultimate in soft water. As far as mains water is concerned "Soft" could be considered as below 50 p.p.m as Calcium Carbonate. In pool water, anything below 250 p.p.m. will have a tendency to dissolve Calcium from other sources. See Calcium Hungry

Spa Essence

An emulsified fragrance compound added to spa baths to give a pleasant aroma and mask the smell of bromine and chlorine sanitisers.

Stabilised Chlorine

Chlorine donors such as "Dichlor" and "Trichlor" which break down slowly and release hypochlorous acid and cyanuric acid. The cyanuric acid inhibits the loss of chlorine in strong sunlight.


Also called Conditioner. See Cyanuric Acid


A regular shock treatment, often used during the times when high bather loads are encountered.




Abbreviation for Total Dissolved Solids

Total Alkalinity

This is a measure of all the alkaline materials in the pool water. In the pH range found in pool waters, the alkalinity is likely to be present in the form of Sodium Bicarbonate. At the correct level, the pH will not alter rapidly as the water is said to be buffered. If the total alkalinity is below 80 p.p.m. then the water will be insufficiently buffered and if it is above 200 p.p.m it will be excessively buffered. In either case the effect of the addition of chemicals will be difficult to control.

Total Chlorine

The total of the (useful) available chlorine and the combined chlorine.

Total Dissolved Solids

This is a measure of everything dissolved in the water, quite a lot of which is undesirable. In particular chlorides and sulphates can corrode pool equipment and surfaces. A total dissolved solids of over 1000mg/l is undesirable and may cause corrosion. The only way to reduce this is by replacement with fresh water. See also Conductivity


A commonly used name for trichloroisocyanuric acid. This is a popular stabilised chlorine donor for domestic pools. Commonly sold in slow dissolving tablets, Trichlor provides 90% of its weight as chlorine.


A situation where suspended particle in the water cause loss of clarity. See Aluminium Sulphate See Clarifier




See pathogenic organisms



Water Balance

The broad overview of the condition of the water taking into account all the factors involved. The water will either be corrosive or scale forming. It is preferable to have the water just on the scale forming side. See Langelier Saturation Index

Water volume

The amount of water in your pool or spa bath. 


The Importance of Water Balance

One of the most important steps of your pool and spa maintenance is maintaining the correct water balance. If this is achieved it will help to avoid a lot of major issues. It helps sanitizing chemicals, chlorine or bromine, to work more effectively. Keeping the correct water balance helps to avoid the water becoming cloudy and the growth of algae. The right water balance will also help to prolong the life of your pool equipment. You should test the water on a regular basis to identify any parameters that are not within the ideal range.

For the ideal water balance we would recommend:

  • Total Alkalinity – 80-150 mg/l
  • pH – 7.2-7.6
  • Calcium Hardness – 80mg-200mg/l
  • Chlorine Reading 2-3 mg/l (Bromine reading 4-6 mg/l)