A chemical that contains hydrogen and can dissolve metals, reacts with bases to form salts, and neutralizes alkaline materials.
Acid Demand:
The amount of acid needed to lower pH to the proper level for pool water.
Microscopic plants deposited in pool or spa water by wind, rain, and dust.
Natural or synthetic substance used for killing, destroying, or controlling algae.
A condition where the pH of water is above 7.0 on the pH scale. Alkaline (often referred to as "base") is the opposite of acidic.


Reversing the flow of water through the filter to clean the elements of the filter.
Microscopic organisms which may be living in the pool water, many of which can cause infection or disease.
A chemical of an alkaline nature that reacts with acids to form salts and neutralizes acidic materials.
Base demand:
The amount of base (or pH increaser) water needs to reach the proper pH range.
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 imbeds itself into porous pool surfaces and can be difficult to completely remove.
A chemical that works to prevent fluctuations in pH.


Calcium carbonate:
Scale that forms on pool surfaces from calcium compounds when pool water is too alkaline, calcium hardness is too high, or total alkalinity is too high.
Calcium Hardness:
The amount of calcium dissolved in water expressed in ppm (parts per million)
Calcium Hypochlorite:
A chlorine compound using calcium as the carrying salt for application.
Chelating Agent:
A chemical compound that ties up iron, copper, or calcium to prevent staining and scaling. Also called a sequestering agent.
Substances formed when chlorine combines with swimmer wastes (nitrogen or ammonia), causing chlorine odor and irritation to skin and eyes.
The most widely used bacteria-killing agent for recreational water treatment. In its elemental form it is a gas and stored in cylinders. Various chlorine compounds are available for pool sanitation, including calcium hypochlorite and chlorinated isocyanurates.
Chlorine Demand:
The chlorine needed to establish a stable, residual chlorine amount for proper sanitation.
Chlorine Residual:
The amount of chlorine readily available after the chlorine demand has been satisfied or not bound up in chloramines.
A product that causes fine suspended particles in water to combine into filterable or vacuumable clusters.
Combined Chlorine:
Chlorine that is chemically bonded to other compounds.
The effect of an acidic environment where pH and/or alkalinity are very low.
Cyanuric Acid:
A chemical compound added to pool water to reduce the degradation of chlorine by the ultraviolet rays of the sun.


Diatomaceous Earth (DE):
A powdery filtering agent composed of the skeletal remains of diatoms (a form of plankton) used in DE filters.
Dichlor (sodium dichloro-s-triazinetrione):
A fast-dissolving chlorine compound with a neutral pH.
The reagent that measures free available chlorine.



Filter Medium:
Sand, DE, or other material used to filter particles out of the water.
A chemical compound added to water causing suspended particles to bond together and sink to the bottom of the pool where they can be vacuumed.
Free Available Chlorine:
Chlorine in pool water that is not combined with ammonia or nitrogenous compounds and is available to sanitize the water.


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.


The amount of calcium and magnesium dissolved in water. It is measured in ppm (parts per million).
An inorganic (unstabilized) family of chlorine compounds used in various forms to provide chlorine for water treatment. Includes calcium hypochlorite and sodium hypochlorite.
Hypochlorous Acid:
The free state of chlorine that actually destroys bacteria and other organic wastes.


Inorganic Chlorine:
Unstabilized chlorine that is vulnerable to degradation by the ultraviolet rays of the sun.
A metal often present in fill water that can make water greenish, yellow, or rust-colored.




The process where improperly balanced pool water can extract minerals from pool surfaces and plaster interiors.
Liquid Chlorine:
Sodium hypochlorite solution used as a disinfectant.


The metals that may be present in water include iron and copper. When either is dissolved in water, the addition of a shock product can turn the water various colors and/or stain the surfaces.
Calcium, manganese, magnesium, nickel, copper, silver, iron, cobalt, and aluminum may be present in water. In high, non-chelated concentrations, minerals can lead to stains and scale.
Mustard Algae:
See Yellow Algae.


Non-Chlorine Shock:
A class of chemical compounds used to oxidize or shock the water without chlorine or bromine.


Organic Chlorine:
A form of chlorine that contains the compound triazinetrione. The most common forms of organic chlorine are dichlor (sodium dichloro-s-triazinetrione) and trichlor (trichloro-s-triazinetrione).
A product that destroys organic and inorganic contaminants such as ammonia, chloramines, and swimmer waste.


A measurement that indicates the acidic or basic nature of the pool water. The pH scale runs from 0 to 14. 7.0 is neutral, a pH below 7.0 is acidic, and a pH above 7.0 is basic. The acceptable range is between 7.2 and 7.8.
Pink Algae:
Not actually an alga, but bacteria that forms colonies with a slimy top layer.
Parts Per Million, a unit of measurement that indicates the amount, by weight, of a chemical in relation to one million parts by weight of water.


Quaternary ammonium algaecides, which are compounds added to water to prevent or kill the growth of algae.


Chemical testing compounds that are used to test for chlorine, bromine, pH, total alkalinity, calcium hardness, etc.
The amount of bromine or free available chlorine remaining in the water.


A chemical agent used to destroy unwanted microorganisms in water.
Mineral deposits that form on pool surfaces and equipment due to excessive calcium in the water.
Sequestering Agent:
A product that ties up minerals tightly in solution, preventing their precipitation, which otherwise forms scale, colors the water, or stains surfaces. Also called a chelating agent.
Shock Treatment:
Adding an oxidizing compound to pool or spa water to chemically break up (oxidize) contaminants like oils, perspiration, and dirt that can interfere with pool sanitation.
Sodium Carbonate:
A powder added to water to increase pH. Also known as soda ash.
Sodium Bicarbonate:
A powder added to water to increase the total alkalinity.
Stabilized Chlorine:
An organic compound of chlorine and cyanuric acid. The most common types are dichlor (sodium dichloro-s-triazinetrione) and trichlor (trichloro-s-triazinetrione).


Test Kit:
A collection of liquid and/or tablet reagents assembled for the purpose of measuring a range of water quality parameters.
>Total Alkalinity (TA):
The amount of alkaline substances present in water. Low total alkalinity can cause metal corrosion, plaster etching, and eye irritation. High total alkalinity causes scale formation, poor chlorine efficiency, and eye irritation.
Total Chlorine:
The sum of both the free and combined chlorine residuals in water.
Total Hardness:
The combined amount of calcium and magnesium hardness in pool water. Trichlor (Trichloro-s-Triazinetrione): A slow-dissolving, organic compound containing 90% available chlorine, typically compressed into sticks and tablets.






Yellow Algae:
A microorganism that 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. Also called mustard algae.