Microscopic plant‐like organisms that contain chloropHyll. Algae are nourished by carbon dioxide (CO2) and use sunlight to carry out pHotosynthesis. It is introduced by rain or wind and grows in colonies producing nuisance masses. Algae are not disease‐causing, but can harbor bacteria, and it is slippery. There are 21,000 known species of algae. The most common pool types and black, blue‐green, green and mustard (yellow or drawn). Pink or red‐colored algae‐like organisms exist but are bacteria and not algae. Maintaining proper sanitizer levels, shocking and superchlorination will help prevent its occurrence.


Also called algicide ‐ A natural or synthetic chemical designed to kill, destroy or control algae.


Also more commonly called total alkalinity. A measure of the pH‐buffering capacity of water. Also called the water’s resistance to change in pH. Composed of the hydroxides, carbonates and bicarbonates in the water. One of the basic water tests necessary to determine water balance.

Automatic Pool Cleaner

A pool maintenance system that will agitate or vacuum debris from the pool interior automatically.

Available Chlorine

The amount of chlorine, both free and combined in the pool water that is available to sanitize or disinfect the water. Some‐ times called residual chlorine.


The process of thoroughly cleaning the filter by reversing the flow of water through it with the dirt and rinse water going to waste.


Single‐celled microorganisms of various forms, some of which are undesirable or potentially disease‐causing. Bacteria are controlled by chlorine, bromine or other sanitizing and disinfecting agents.


A chemical or element that kills, destroys or controls bacteria.

Balanced Water

The correct ratio of mineral content and pH level that prevents the water from being corrosive or scale forming.


By‐products formed when bromine reacts with swimmer waste (perspiration or urine), nitrogen or fertilizer. Bromamines are active disinfectants and do not smell, although high levels are body irritants. Bromamines are removed by superchlorination or shock treating.


A common term for a bromide salt used to supply bromide ions to the water so they may be oxidized or changed into hypobromous acid, the killing form of bromine. Used as a disinfectant.


A mechanical or electrical device for dispensing bromine at a controlled rate. Most often a canister or floater filled with tablets of bromine.


A common name for a chemical compound containing bromine that is ued as a disinfectant to destroy bacteria and algae in swimming pools and spas. Available as a tablet or as sodium bromide, a granular salt.


Abbreviation for British Thermal Unit. The amount of heat necessary to raise 1 lb. of water 1 degree Fahrenheit.


An arrangement of pipes, gates and valves by which the flow of water may be passed around a piece of equipment or diverted to another piece of equipment; a controlled diversion.

Calcium Carbonate

Crystalline compounds formed in swimming pool and spa water when the calcium, pH and total alkalinity levels are too high. Once formed, the crystals adhere to the plumbing, equipment, pool walls and bottom. These crystals are better known as scale.

Calcium Chloride

A soluble white salt used to raise the calcium or total hardness level in the pool or spa.

Calcium Hardness

The calcium content of the water. Calcium hardness is sometimes confused with the terms water hardness and total hardness. Too little calcium hardness and the water is corrosive. Too much calcium hardness and the water is scale forming. One of the basic water tests necessary to determine water balance. Minimum level is 150 ppm. Ideal range is 200 to 400 ppm.


A replaceable porous element made of paper or polyester used as the filter medium in cartridge filters.

Cartridge Filter

A pool or spa water filter that uses a replaceable porous element made of paper or polyester.

Chlorine Neutralizer

A chemical used to make chlorine harmless. Used in test kits to counteract the bleach‐ ing effect of the chlorine or bromine in order to increase the accuracy of pool water tests. Sold as chlorine and bromine neu‐ tralizer, it is used to destroy excessive amounts of chlorine or bromine, so the high levels will not affect swimmers.


A mechanical or electrical device for dispensing chlorine at a controlled rate. Most often a canister or floater filled with tablets of chlorine.


A term used to describe any type of chlorine compound used as a disinfectant in swimming pool and spa water or to kill, destroy or control bacteria and algae. In addition, chlorine oxidizes ammonia and nitrogen compounds (swimmer and bather waste).

Chlorine Demand

The amount of chlorine necessary to oxidize all organic matter (bacteria, algae, chloamines, ammonia and nitrogen compounds) in the pool or spa water.

Chlorine Generator

An electrical device that generates chlorine from a salt solution in a tank or from salt added to the pool water.

Chlorine Lock

This is a term that implies that an over‐abundance of cyanuric acid (stablizer or conditioner) in the water would cause the chlorine to be all "locked up." This is not true.


The degree of transparency of the water.

Combined Chlorine

Undesirable, foul‐smelling, body‐irritating compounds formed when insufficient levels of free available chlorine react with ammonia and other nitrogen‐containing compounds (swimmer and bather waste, fertilizer, perspiration, urine, etc.). Combined chlorine is still a disinfectant, but it is a much weaker, ineffective form of chlorine.


The cap or top lip on the pool or spa wall that provides a finished edge around the pool or spa. It can be formed, cast in place or precast, or prefabricated of extruded aluminum or rigid vinyl. It may also be part of the system that secures a vinyl liner to the top of the pool wall.

Coupling A

Plumbing fitting that is used to connect two pieces of pipe.

Cover, Hard‐Top

A cover used on pools, spas and hot tubs that rests on the lip (coping) of the pool or spa deck ‐ not a flotation cover. Used as a barrier to swimmers and bathers, and for maintenance and thermal protection.

Cover, Solar

A cover that, when placed on the water’s surface of a pool, spa or hot tub, increases the water temperature by absorption and transmission of solar radiation; reduces evaporation and prevents wine‐borne debris from entering the water.

Cover, Winter

A cover that is secured around the perimeter of a pool, spa or hot tub that provides a barrier to bathers and debris when the pool, spa or hot tub is closed for the season.

D. E. Filter

Diatomaceous Earth Filter ‐ A filter designed to use diatomaceous earth (D.E.) as the filter medium. The D.E. is added through the skimmer with the pump on, which takes the D.E. and deposits it on a grid. The D.E. then becomes the filter medium.


Those areas immediately adjacent to a pool, spa or hot tub that are specifically con‐ structed or installed for use by bathers for sitting, standing or walking.

Diatomaceous Earth

Also called D.E. ‐ A white powder composed of fossilized skeletons of one‐celled organisms called diatoms. The skeletons are porous and have microscopic spaces. The powder is added through the skimmer with the pump on and deposits itself on a grid. The powder then becomes the filter medium.

Diving Board

A recreational mechanism for entering a swimming pool, consisting of a semi‐rigid board that derives its spring from a fulcrum mounted below the board and attached to the deck.


An electrochemical reaction causing a black stain normally found around metal fixtures or on the plaster. It is caused by two dissimilar metals being plumbed together or from an improper electrical grounding of pool equipment or lights. Electrolysis also means the decomposition of water and other inorganic compounds in aqueous solution by means of electricity. Chlorine generators use this principle to produce chlorine from salt in the water.


Finespun filaments of glass which are avail‐ able in a rope or mat form. When used in a process with polyester resins, catalysts and hardeners, can be formed or molded into pools, spas and related shapes.

Filter Cartridge

A replaceable porous element made of paper or polyester used as the filter medium in cartridge filters.

Filter Cycle

The operating time between cleaning or backwashing cycles of a filter. Also the amount of time the filter has water flowing through it each day expressed in hours.

Filter, Sand

A type of filter media composed of hard, sharp silica, quartz or similar particles with proper grading for size and uniformity. The most common grade used is No. 20 in sand filters.

Filtration Rate

The rate at which the water is travelling through the filter, expressed in U.S. gallons per minute (gpm) per square foot of filter area.

Flow Rate

The quantity of water flowing past a design‐ ated point within a specified time, such as the number of gallons flowing past a point in 1 minute ‐ abbreviated as gpm.


A froth of bubbles on the surface of the water. Usually comes from soap, oil, deo‐ dorant, hair spray, suntan oil, etc., that is shed into the water as swimmers enter.

Free Available Chlorine

Free Available Chorine ‐ The amount of free chlorine in the pool or spa water that is available to sanitize or disinfect the water. Sometimes called residual or available chlorine.


An abbreviation for gallons per minute.


The amount of calcium and magnesium dissolved in the water. "Water" or "total" hardness refers to the total magnesium and calcium dissolved in the water. Calcium hardness refers to just the calcium. Measured by a test kit and expressed as ppm. The proper range is 200 to 400 ppm.

Heat Exchanger

A device located inside the heater providing for the transfer of heat from the heat source to the water. This is usually a seriew of metallic tubes with fins located just above the flames.


A fossil‐fueled, electric or solar device used to heat the water of a pool, spa or hot tub.

Hot Tub

A spa constructed of wood with the sides and bottom formed separately and joined together by hoops, bands or rods.


The rotating member of a pump. The part of the pump that moves the water.


A fitting in the pool or spa on the water return line from the equipment that water returns to the pool. Usually the last thing on the return line.


A water‐sanitation device that uses electricity to generate metal ions, which are dispersed in the water. It works by passing a low‐voltage DC current through a set of metallic (usually copper and silver) electrodes placed in line with the circulation equipment. The copper is an algaecide, while the silver is a bactericide. Does not remove swimmer waste.


A brand name and registered trademark for a specific line of spas and whirlpools.


A structure for climbing up or down; consists of two parallel sides joined by a series of crosspieces that serve as footrests. It is used for getting in and out of the pool. A double‐access ladder straddles the pool wall of an above‐ground pool. An in‐pool ladder is located in the pool only.

Light Niche

The area in a pool or spa that house the underwater light.


Also called vinyl line ‐ The vinyl membrane that acts as the container to hold or contain the water.

Liquid Chlorine

A sodium hypochlorite solution. Usually provides 10 to 12% available chlorine; has a pH of 13 and requires that small amounts of acid be added to the pool to neutralize the high pH. Good for regular chlorination and superchlorination.

Main Drain

This term usually refers to a plumbing fitting installed on the suction side of the pump in pools, spas and hot tubs. Sometimes called the drain and is located in the deep‐ est part of the pool, spa or hot tub. It is not a drain, such as a drain on a kitchen sink. Main drains do not allow the water to drain to waste but rather connect to the pump for circulation and filtration.

Muriatic Acid

(31.45% hydrochloric acid) ‐ Also called liquid acid ‐ An acid 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.

Non‐Chlorine Shock

A term given to a class of chemical compounds that are used to oxidize or shock the water (destroy ammonia, nitrogen and swimmer waste). They contain no chlorine or bromine and do not kill living organisms. Swimmers may re‐enter the water in only 15 minutes after adding a non‐chlorine shock.


A gaseous molecule comprised of 3 atoms of oxygen. It is generated on site from air or oxygen and used for oxidation of water contaminants.


A term used to indicate the level of acidity or alkalinity of pool water. Too low of pH causes etched plaster, metal corrosion and eye irritation. Too high of pH causes scale formation, poor chlorine efficiency and eye irritation. The ideal range for pH in swimming pools is 7.4 to 7.6.


A mechanical device, usually powered by an electric motor, which causes hydraulic flow and pressure for the purpose of filtration, heating and circulation of pool and spa water. Typically, a centrifugal pump is used for pools, spas and hot tubs.


This usually refers to the filter medium used by a sand filter. The grade most often specified by filter manufacturers is grade No. 20 with a particle size of 45 to 55 mm (millimeters).

Sand Filter

A filter using sand or sand and gravel as the filter medium.

Shock Treat

The practice of adding significant amounts of an oxidizing chemical ‐ (usually non‐ chlorine oxidizers, such as sodium persulfate or potassium peroxymonosulfate) ‐ to the water to destroy ammonia and nitrogen com‐ pounds or swimmer waste.


A device installed through the wall of a pool or spa that is connected to the suction line of the pump that draws water and floating debris in the water flow from the surface without causing much flow restriction.

Skimmer Basket

A removable, slotted basket or strainer placed in the skimmer on the suction side of the pump, which is designed to trap floating debris in the water flow from the surface without causing much flow restriction.

Skimmer Weir

Part of a skimmer that adjusts automatically to small changes in water level to assure a continuous flow of water to the skimmer. The small floating "door" on the side of the skimmer that faces the water over which water flows on its way to the skimmer. The weir also prevents debris from floating back into the pool when the pump shuts off.

Sodium Bicarbonate

(Baking Soda or Bicarb) ‐ A chemical used to raise total alkalinity in pool and spa water with only a slight affect on the pH.

Soft Water

Water that has a very low calcium and magnesium content (water hardness) ‐ usually means less than 100 ppm or 6 grains. Also water that has gone through a water softener. Pools and spas should never be filled with soft water from a softener. Water with less than 100 ppm of hardness should be increased to a minimum of 150 to 200 ppm using calcium chloride.

Solar Cover

A cover that, when placed on the water’s surface of a pool, spa or hot tub, increases the water temperature by absorption and transmission of solar radiation; reduces evaporation and prevents wind‐borne debris from entering the water.

Solar Heating System

Solar Heating System ‐ It is usually panels or coils of plastic or metal through which water passes to increase the temperature from the sun’s radiant heat.

Stabilized Chlorine

A family of chlorine pool sanitizers that contain conditioner (cyanuric acid or iso‐ cyanuric acit) to protect the chlorine from the degrading UV rays in sunlight. Most common types are sodium dichlor and trichlor. The granular form is dichlor which is fact‐ dissolving and can be used for regular chlorination or superchlorination by broad‐ casting into the pool or spa. Tablet or stick form is trichlor (which is usually used in a chlorine feeder ‐ either the floating type or in‐line erosion type) used for regular chlorination only.

Super Chlorination

The practice of adding an extra large dose (5 to 10 ppm) of chlorine to the water to destroy ammonia, nitrogen and swimmer waste, which can build up in the water. This level of chlorine is required to destroy all of the combined chlorine in the water, which is called breakpoint chlorination.

Test Kit

An apparatus or device used to monitor specific chemical residuals, levels, constituents or demands in pool or spa water. Kits usually contain reagents, vials, titrants, color comparators and other materials needed to perform tests. The most common pool and spa water tests are: pH, total alkalinity, free available chlorine, water hardness, cyanuric acid, iron and copper.

Total Alkalinity

The total amount of alkaline materials pre‐ sent in the water. Also called the buffering capacity of the water. It is the water’s resistance to change in pH. Low total alkalinity causes metal corrosion, plaster etching and eye irritation. High total alkalinity causes scale formation, poor chlorine efficiency and eye irritation.

Total Chlorine

The total amount of chlorine in the water. It includes both free available and combined chlorine.

Underwater Light

A fixture designed to illuminate a pool or spa from beneath the water’s surface.

Vinyl Liner

The vinyl membrane that acts as the container to hold or contain the water.