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a liquid or dry compound used to reduce the pH of swimming pool or spa water. See also "muriatic acid" and "sodium bisulfate".

Acid Demand

the amount of acid (or pH decreaser) water needs to reach the proper pH range. For example, to lower pH from 8.0 to 7.6, your pool may "demand" 2 quarts of acid. The actual amount needed will be dependent upon pool volume and other factors. Some test kits contain the titration test used to determine acid demand.


microscopic plant-like organisms that contain chlorophyll (green coloration) and are nourished by sunlight and carbon dioxide. Rain and wind can introduce algae to pools, where it is capable of rapid reproduction. There are 21,000 known species of algae, but only a few are known to grow in pools such as green, blue-green or black, brown and yellow-green (mustard). Algae blooms may form separate spots, or seem to grow in sheets. See also "green algae", "yellow algae", "black algae", "pink algae"


natural or synthetic substances used for killing, destroying, or controlling algae. Algaecides perform best when used regularly as part of a normal maintenance program along with a routine sanitization program. A variety of algae treatment products are available including polymers, quat compounds, chlorine enhancers, copper and silver compounds, and herbicides.


See Total Alkalinity


short for aluminum sulfate, this powdered substance is used as a flocculent to attract suspended particles in the water together so they sink and can then be vacuumed. It is used to clear cloudy water. A small amount of alum is occasionally used as a sand filter additive.

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microscopic organisms, some of which are harmful to bathers. Some are pathogens, which can cause infectious diseases if not treated with a sanitizer.


a substance such as chlorine that kills bacteria. Silver algaecides are actually bactericidal, and are useful on "pink algae"


a term used to describe water that has all of its chemistry parameters in their proper ranges and is thus neither scaling nor corrosive. The key components of water balance are pH, total alkalinity, calcium hardness, and temperature.


chemicals of alkaline nature that will counteract the pH of an acid, eventually neutralizing at 7.0. Common bases used in pools include soda ash, sodium bicarbonate, and sodium carbonate.

Base Demand

the amount of base (or pH increaser) water needs to reach the proper pH range. For example, to raise pH from 7.2 to 7.6, a pool may "demand" 2 cups of soda ash. The actual amount depends on pool volume and other factors. Some test kits contain the titration test used to determine base demand.

Bather Load

the number of people in the pool or spa at any given moment, or during any stated period of time. Bather load also is affected by lotions, deodorants, perfumes, etc.


see "sodium bicarbonate"


a type of sanitizer using the polymer PHMB in place of the more common halogen-type sanitizers, chlorine and bromine.

Black Algae

a type of algae that grows on pool walls and floors as dark spots. Colonies usually form in areas with less circulation. Black algae feels slimy and can be brushed off with some effort. The algae embeds itself into porous pool surfaces and can be difficult to completely remove.

Breakpoint Chlorination

the application of a large dose of chlorine intended to reach the point where free chlorine, measured in ppm, is at a high enough level to break apart molecular bonds, specifically the combined chlorine molecules. When this point is reached with sufficient addition of chlorine, undesirable compounds responsible for odors, eye burn, irritation and poor sanitizing in the pool are oxidized.


combined bromine-nitrogen molecules, which form as bromine sanitizes. Unlike chloramines, which are strong smelling and offer no sanitizing properties, bromamine compounds continue to sanitize.


a salt that contains bromide ion (Br-). Bromide becomes hypobromous acid when oxidizers such as chlorine, ozone, or persulfates are added.


a member of the halogen family, commonly used as a sanitizer in spas, because it is effective in a wide range of water temperatures and pH levels. Less commonly used for pools, bromine is both an oxidant and a biocidal agent.


a base compound such as sodium bicarbonate (baking soda) added to water to increase alkalinity and therefore minimize the effect of pH fluctuations on the pool.

Buffering Capacity

the ability of the pool water to resist changes in pH. If pH bounces, or resumes unwanted levels soon after adjustment, the buffering capacity is too low and a buffer or pH locking product should be used.

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Calcium Carbonate

crystalline deposits of calcium known as scale that may form on surfaces, equipment, or inside pipes and heaters when water is not kept in chemical balance.

Calcium Chloride

a flaked calcium salt used to raise levels of calcium hardness in pool or spa water.

Calcium Hardness

the amount of calcium dissolved in water, expressed in ppm. A titration test is used to determine whether the level is within the proper range to achieve balance. If it's too low, the water is corrosive - while too much calcium hardness can lead to cloudy water or produce scale.

Calcium Hypoclorite

cal hypo for short, this chlorine-calcium compound is sometimes used as a disinfectant for routine sanitization or for superchlorination.

Carbon Dioxide

a naturally occurring gas, which when present in water, provides necessary food for the growth of algae.


a salt that contributes to the make up of total alkalinity, scale and TDS.

Chelating Agent

a water soluble molecule that can bond tightly with metal ions, keeping them from coming out of suspension and depositing as stains and scales on pool surfaces and equipment. See also "sequestering agent"

Chemical Feeder

a mechanical device for applying chemicals to pool or spa water.


compounds that form when free chlorine combines with nitrogen-containing compounds such as perspiration, urine and cosmetics. Chloramines can cause eye and skin irritation, have strong objectionable odors and reduce sanitizing capability. See also "combined chlorine"


a device that adds a chlorine sanitizer at a controllable rate.


a member of the halogen family of sanitizers, used in swimming pools as a gas, liquid, granular or tablet compound. When added to water, it acts as an oxidizer, sanitizer, disinfectant and biocide. Chlorine is the most popular pool and spa disinfectant. See also "combined chlorine", "free chlorine" and "total chlorine".

Chlorine Demand

the amount of a free available chlorine needed to destroy organic contaminants and establish a chlorine residual for effective sanitization.

Chlorine Residual

the amount of free chlorine in the water.


a product that causes fine suspended particles in water to combine into filterable or vacuumable clusters.


the degree to which an object can be seen through a given depth of water.


a chemical used to gather and precipitate a suspended material that otherwise may make pool or spa water cloudy and settle it to the bottom of the pool for vacuuming.

Combine Chlorine

one of the two forms of chlorine, combined chlorine is the form that has been used up and thus no longer has the ability to disinfect. To measure combined chlorine, subtract a free available chlorine test result from the result of a total chlorine test. There is no test for combined chlorine. An overabundance of combined chlorine causes the eye irritation and odor often associated with chlorine. Adding a large dose of chlorine or a non-chlorine shock can correct this condition. See also "free chlorine", "chloramines", "superchlorination" and "shock"


a metal found in corrosive water or if copper-based algaecides are over-dosed. Copper generally gives the water a green or blue cast. Copper commonly forms blue-green to black colored staining on pool surfaces. Copper is also associated with green hair and fingernails.

Copper Sulfate

a granular form of copper, sometimes called Bluestone, used to treat algae in natural bodies of water. It is not suggested for use in swimming pools or spas as it stains surfaces readily.


the effect of an acidic environment, where pH and/or alkalinity are very low. Corrosion can lead to surface etching or pitting, as well as damage to equipment and plumbing.

Cyanuric Acid

a granular chemical that shields chlorine in the water from being destroyed by sunlight. Also called conditioner and stabilizer. Not needed in indoor pools or when using a stabilized form of chlorine.

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a fast-dissolving granular compound containing chlorine and cyanuric acid (stabilizer or conditioner). It has a neutral pH and is quick-dissolving, so it can be used for regular chlorinating or superchlorination.


a chemical such as chlorine, bromine or silver that kills living organisms such as bacteria that contaminate pool and spa water.


a form of test kit reagent used to determine chlorine levels in pool or spa water. It is available as premeasured tablets or drops. Unlike OTO, DPD testing allows determination of both total and free available chlorine levels, which, through subtraction, gives the combined level. (Total minus free equals combined.)

Dry Acid

a dry white crystal product used to lower pH and total alkalinity by producing acid when added to water. Considered safer to handle and less caustic than muriatic acid. See also "sodium bisulfate"

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used in swimming pool and spa products to break down and digest oils and grease.

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see free chlorine


a chemical such as alum that causes suspended material and/or algae to clump together and sink to the pool floor for vacuuming. See also "coagulant."


usually caused by high TDS levels working in combination with soft water and oils. Defoamer products effectively control foam.

Free Chlorine

one of the two forms of chlorine typically found in pool water, free chlorine is the active form that is able to destroy bacteria, algae and other potentially harmful organisms as they enter the water. The proper amount of free chlorine is important because it ensures ongoing, active protection for swimmers. Also called free available chlorine and often abbreviated in test kit/strip instructions as FAC. See also "combined chlorine."

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Green Algae

a free-floating organism that turns water cloudy and green. This type of algae is the most common and easiest to clear up. Green algae exerts a tremendous chlorine demand, however, so the more that can be removed through filtration, and the use of a quality algaecide, the easier it will be to establish an effective chlorine residual.

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a member of the family of elements bromine, chlorine, fluorine and iodine. The first two are common spa and pool disinfectants.

Hard Water

water that has high levels of calcium hardness and other salts.


the amount of calcium and magnesium dissolved in water. It is measured in parts per million (ppm).

Hydrogen Peroxide

an unstable, colorless, heavy liquid commonly used as a bleach in industry and as an antiseptic in households. It is used as an oxidizing or shock treatment in biguanide sanitized pool water.

Hypobromous Acid

the active sanitizing form of a bromine-based water treatment process. (see hypochlorous acid)


a family of chlorine compounds such as calcium hypochlorite and lithium hypochlorite, both granular, and the liquid sodium hypochlorite. When these compounds contact water, they release hypochlorous acid, the active sanitizing agent.

Hypochlorous Acid

a chlorine-based water treatment process and the desired, active sanitizing agent.

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a device through which water flows and receives charged metal ions, usually copper, zinc and/or silver. Copper is an algaecide and algaestat, while silver and zinc are known for their properties as a bactericide. Ionizers may be used as part of a water treatment program along with chlorine or bromine.


a metal often present in fill water that will give water a greenish, yellow cast or rust-colored cast. It can also stain surfaces. Most frequently it is found naturally occurring in fill water or due to corrosive water (low pH). A sequestering agent or a chelating agent can prevent colored water or staining.

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Langelier Index

see saturation index

Liquid Chlorine

a sodium hypochlorite solution that usually provides about 10 percent available chlorine, has a pH of 13 and requires that small amounts of acid be added to the pool to neutralize the high pH.

Lithium Hypochlorite

a fast-dissolving white solid used as a sanitizer and oxidizer in pools, typically containing 35 percent available chlorine.

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the metals that may be present in water include iron and copper. When either is dissolved in the water, the addition of a shock product can turn the water various colors and/or stain the surfaces.


the minerals that may be present in water include calcium, manganese, magnesium, nickel, copper, silver, iron, cobalt and aluminum. In high, non-chelated concentrations, minerals can lead to stains and scale when conditions are right. Such conditions are high pH and/or total alkalinity.

Muriatic Acid

also called liquid acid, this dilution of hydrochloric acid can be used to reduce the pH and alkalinity levels in pool water. It is also used in acid washing, a process that removes stains and scale from pool plaster. It is extremely aggressive and corrosive.

Mustard Algae

see yellow algae

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a chemical used in test kits to counteract the bleaching effect of chlorine or bromine so the tests will be accurate.


when combined with chlorine, nitrogen creates chloramines, which cause odors and skin and eye irritation. Nitrogen can be found in perspiration, suntan oil, hair tonics, etc.

Non-Chlorine Shock

a class of chemical compounds used to oxidize or shock the water without chlorine or bromine. They are not sanitizers and therefore swimmers may re-enter the water in only 15 minutes after adding a non-chlorine shock. Their main ingredient is usually potassium monopersulfate and it comes in a dry granular form.

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Organic Matter

carbon-based substances, generally originating from living organisms, often introduced to pools or spas by bathers and the environment.


a form of test kit reagent used to test for total chlorine levels. This test does not differentiate between free (desirable form of chlorine) and combined chlorine. Used on test strips and in traditional liquid test kits.


the chemical destruction of organic waste and compounds in water. The same term also refers to discoloration due to metals that may form on pool or spa surfaces if water is corrosive. Rust is a form of this kind of oxidation.


see shock treatment


a product that destroys organic and inorganic contaminants such as ammonia, chloramines and swimmer waste in water. See also "shock treatment."


the molecule containing three atoms of oxygen; known to be a very powerful but short-lived oxidizer. See "ozone generator."

Ozone Generator

a device that uses UV radiation or corona discharge technology to produce ozone, which then oxidizes any contaminants it can contact before dissipating.

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an index of the alkalinity or acidity of the water, where 7.0 is neutral, above 7 is basic or alkaline and below 7 is acidic. A high pH level (too alkaline) can irritate the eyes and skin, produce mineral scaling on pools and reduce the sanitizing effect of chlorine. A low pH level (too acidic) can also cause swimmer discomfort as well as corrosion of fixtures and equipment. The ideal range is 7.4-7.6.

Pink Algae

not acually an algae, but rather a bacteria that forms colonies with a slimy top layer. It is best treated with heavy doses of chlorine or with specialized products for treating this contaminant. Algaecides are generally not very effective for treating this condition.


the term itself means mixture. In the context of pool products, it most commonly refers to a type of algaecide or water clarifier made up of chains of repeating molecules.

Potassium Monopersulfate

See non-chlorine shock


short for parts per million - a simple way to measure chemical concentration.

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short for quaternary ammonium compound - a type of algaecide composed of ammonia compounds. It is an effective algaestat for green and blue/green algae.

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a tablet, powder or liquid material used in water testing.


the amount of bromine or free available chlorine remaining in the water.

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Salt Water Chlorine Generator

produces chlorine in swimming pools by the process of electrolysis. Water containing a low concentration of salt passes over the chlorine generator cell and produces chlorine that is instantaneously transformed into hypochlorous acid. See also hypochlorous acid, salt water pool

Salt Water Pool

a pool that utilizes a salt water chlorine generator to produce chlorine. It is important that in addition to the chlorine generator specialty chemicals are used to maintain proper water balance. See also salt water chlorine generator


to kill disease-causing organisms


a chemical agent used to destroy unwanted microorganisms in water.

Saturation Index

a system devised by Dr. Wilfred Langlier that determines water balance by assigning values to levels of pH, total alkalinity, calcium hardness and water temperature. When these parameters are in balance, the water will neither be corrosive or scaling.


usually whitish in color, scale forms on surfaces and equipment when mineral salts are forced out of solution. A scaling condition is one in which calcium hardness, pH and/or alkalinity levels are out of balance. Scale may appear as gray, white or dark streaks on plaster, fiberglass or vinyl. It may also appear as a hard crust around tile.

Sequestering Agent

a product that ties up minerals tightly in solution, preventing their precipitation, which otherwise form scale, color the water or stain the pool or spa. See also "chelating agent."


a product used in shocking, such as hypochlorites, potassium monopersulfate or hydrogen peroxide. Also the act of shocking. See "shock treatment."

Shock Treatment

adding significant amounts of an oxidizing compound to pool or spa water to chemically break up (oxidize) contaminants such as suntan oils, cosmetics, perspiration and windblown dirt.

Soda Ash

a basic chemical used to counteract an acidic condition by raising pH in pool and spa water.

Sodium Bicarbonate (Baking Soda)

a base used to increase alkalinity.

Sodium Bisulfate

a granular form of acid, used to lower pH and/or total alkalinity. 2.5 lb of dry acid are equal to 1 qt of muriatic acid.

Sodium Carbonate

the chemical term for soda ash. It is a white powder used to raise the pH.

Sodium Dichlor

see dichlor

Sodium Hypochlorite

a form of liquid chlorine that is about 10 percent chlorine. Can be used in pools.

Sodium Thiosulfate

a chemical used to neutralize chlorine.

Soft Water

water with low levels of dissolved calcium and magnesium.

Stabilized Chlorine

a form of chlorine that has been blended with a stabilizer so it can remain active in strong sunlight. see also "stabilizer," "dichlor" and "trichlor."


see cyanuric acid.

Stain Remover

a chemical or chemical system that can be added to the water or applied to surfaces to remove existing stains.See also "chelating agent" and "sequestering agent."


the addition of a sufficient amount of chlorine to reduce cloudy water, slime formation, musty odors, algae and bacteria counts, and/or improve the ability to maintain sanitizer residuals. Superchlorination is one method of shock treatment. See also "shock treatment" and "breakpoint chlorination."

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see total dissolved solids

Test Kit

a collection of liquid and/or tablet reagents assembled for the purpose of measuring a range of water quality parameters.

Test Strip

a small strip of paper to which reagent-coated blocks are attached, providing a one-step method of testing multiple water quality parameters by dipping the strip into pool or spa water and evaluating the resulting color on the blocks.


a method of testing for total alkalinity, calcium hardness and acid/base demand by adding a titrant drop by drop, until a color change is observed.

Total Alkalinity (TA)

the amount of alkaline substances (carbonates and bicarbonates) present in water. Also called the buffering capacity of the water because these substances contribute to the water's ability to resist change in pH. Low total alkalinity (or acidic water) causes metal corrosion, plaster etching and eye irritation and chlorine loss. High total alkalinity causes scale formation, poor chlorine efficiency and eye irritation. It is measured in ppm with most standard test kits and strips. See also "pH."

Total Chlorine

the sum of both the free and combined chlorine residuals in water. This is one of the levels that can be determined with a DPD test. The difference between it and the level of free avaiable chlorine is the level of combined chlorine.

Total Dissolved Solids (TDS)

the amount of dissolved matter in water. TDS rises every time chemicals are added to water. The only way to effectively lower TDS is to drain part or all of the water and replace it with fresh water.

Total Hardness

the combined amount of calcium and magnesium hardness in pool or spa water. When total hardness is too high, scale can form, causing pool filters or plumbing to clog. The water may also appear cloudy. Water that is too soft (has low hardness level) will slowly disslove plaster walls and corrode metal fixtures.


a stabilized, slow-dissolving, tablet or granular form of chlorine that provides 90 percent available chlorine. Can be used for regular chlorinating, but must be dispensed using a feeder because it is very acidic.


the level of water cloudiness due to microparticle suspension.

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Water Balance

see balance


preparing a pool or spa for freezing weather using a combination of water care products and equipment protection methods. Depending on the climate, this may involve disconnecting equipment, adding pool antifreeze to pipes, or simply using a winter-strength algaecide and sanitizer dose.

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Yellow Algae

sometimes called mustard algae, this microorganism appears on pool walls as a fine dust. Typically it is seen first on surfaces that don't receive direct sunlight. This algae is easy to brush off, but it frequently returns. Most pool experts agree that this type is the most difficult algae to control, but there are several products designed to combat it specifically.

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