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Pool care has a language all its own. We've provided this glossary to help you understand some common pool care terms. Select a letter below to view a term.

A ¦ B ¦ C ¦ D ¦ F ¦ G ¦ H ¦ I ¦ L ¦ M ¦ O ¦ P ¦ Q ¦ S ¦ T ¦ W

Acid Demand

The amount of acid required to lower the pH and total alkalinity of pool water to the recommended level.


The state of being acidic (low pH) as opposed to being basic (high pH); measuring below neutral 7.0 on the pH scale.

Aggressive Water

A description of unbalanced water due to low pH, low alkalinity, and/or low calcium hardness. Aggressive water attacks and corrodes pool surfaces, metal fixtures and metal plumbing.


Microscopic forms of plant life that can inhabit a pool or spa. Some strains are free-floating; others cling to the pool or spa's surfaces. Algae spores thrive on sunlight, display a variety of colors and some strains can damage the pool's surface.

Algaecide or Algicide

Chemical compounds added to the pool to kill algae.


See Total Alkalinity.


Aluminum sulfate. A compound used to flocculate and sink small suspended particulate matter for removal by filtration or vacuuming.

Available Chlorine

See Free Available Chlorine.



Reversing the flow of water through the filter to clean the filter medium or elements. This is typically found on sand and D.E. filters. Cartridge filters are removed and hosed off to remove debris.


Small, single cell organisms that contaminate a pool or a spa. Bacteria can be introduced from the environment and by swimmers.

Balanced Water

Water with the correct ratio of mineral content and pH levels that prevents it from becoming too acidic (corrosive) or too alkaline (scale forming).

Baking Soda

See Sodium Bicarbonate.


The state of being basic (alkaline), as opposed to being acidic; also, known as measuring above 7.0 on the pH scale.


Dispersing  a pool chemical across a wide area of the pool surface.


A member of the halogen family. Compounds of bromine are used regularly to sanitize spas.

Bromide Reserve/Residual

The bromide reserve is a bank of sodium bromide that is created when using hth Spa™ Brom-Start. Bromide is not a sanitizer. Bromide must be activated with an oxidizer, such as calcium hypochlorite or potassium monopersulfate, to form an active bromine. Since the brominating tablets are slow dissolving, a quick way to form a immediate residual of active bromine to begin sanitization is to add sodium bromide and then oxidize it to activate bromine.


Calcium Chloride

A compound used to raise calcium hardness levels in pools and spas.

Calcium Hardness

The amount of dissolved calcium in the pool or spa water expressed as ppm or parts per million.

Calcium Hypochlorite

A calcium based chlorine sanitizer and oxidizing compound. The sanitizing or chlorinating agent in hth® Chlorinating Granules and hth® 3-in-1 Chlorinating Skimmer Tablets.


Substances formed when chlorine combines with nitrogen compounds such as urine, perspiration and body oils, cosmetics, lotions, etc.


A product that adds available chlorine to pool water.


Member of the halogen family effectively used in swimming pool and spa sanitation. It's available as a gas or compound as a liquid, various sized tablets and granular forms. Can act as a sanitizer, oxidizer, algaecide and shocking compound. This is the sanitizing element in products such as hth® Shock 'N Swim.

Chlorine Demand

The amount of chlorine needed to completely kill bacteria and algae,  and to oxidize other pool contaminants.

Chlorine Lock

A term that is sometimes used to refer to overstabilization (see entry for overstabilization).

Chlorine Residual

The amount of free available chlorine in the water.


A chemical used to coagulate particles suspended in pool water, making these particles easier for the filter to remove and clear the water.

Combined Chlorine (CC)

The portion of total chlorine existing in water in chemical combination with ammonia, nitrogen, and/or organic compounds; mostly compromised of chloramines and ineffective for sanitation. The difference between free available and total chlorine levels in pool water.


Deterioration of fixtures, surfaces, or plumbing.

Corrosive Water

Generally refers to a water condition of low pH (acid condition) that can corrode metal pipes, pool fixtures and pumps. Corrosive water can also etch plaster and cause eye irritation.

Cyanuric Acid (CYA)

See Stabilizer.


Diatomaceous Earth

A dry, powder-like filter media composed of microscopic fossilized diatoms; diatomite, D.E.

D.P.D. #1, #3, #4

Reagents used in the determination of free available chlorine; combined chlorine and total chlorine; Diethyl-p-phenylenediamine.



A mechanical device that dispenses chemicals into a pool or spa.


A device that removes undissolved particles from water by recirculating the water through a porous substance (a filter medium or element).

Filter Medium

The material such as sand, D.E., cartridge fabric, and zeolite through which the pool or spa water passes to remove suspended particles.


A device floating on the surface of the pool or spa water containing a supply of sanitizer, usually in tablet form that is fed into the water over a period of time.

Floccing or Floc Treating

Using an agent  that causes suspended solids in the water to stick together and settle to the bottom to be removed by vacuuming.

Free Available Chlorine (FAC)

The portion of total chlorine remaining in chlorinated water that is not combined with ammonia or nitrogen compounds and will react chemically with undesirable or pathogenic organisms; the amount of hypochlorous acid and hypochlorite ion in the water.



A type of concrete pool construction; pneumatically applied cement/sand mixture.



A chemical family that includes flourine, chlorine, bromine, iodine, and astatine. Chlorine and bromine are commonly used to sanitize pool and spa water.


The amount of calcium and magnesium dissolved in water epxressed as ppm.

High Dissolved Solids

Pool water containing high levels of Total Dissolved Solids (see entry for TDS). High levels of dissolved solids may cause water to have a "flat" or "salty" taste or cause water to appear "dull" or "dead". Pools with water three to five years old, or those containing water with solids higher than 2,000 ppm should be drained or diluted with fresh water.

Hydrochloric Acid

A strong acid used in swimming pools to lower pH and total alkalinity. Commonly known as Muriatic Acid.



Family of pool chlorinator products that include sodium dichloroisocyanurate and trichloroisocyanurate. These products contain cyanuric acid in their structures and are considered "self-stabilizing". Available in tablet and granular form. 


Langelier Index (LSI)

A numerical calculation based on pH, total alkalinity, total dissolved solids, and water temperature that is used to predict the tendency of water to cause to cause scale formation.

Liquid Chlorine

See Sodium Hypochlorite; often called bleach.



A mixture of white Portland cement and white marble aggregate used as a finish coat for pools; Marbleite.

Muriatic Acid

See Hydrochloric Acid.


Orthotolidine (OTO)

An indiciator reagent that turns yellow in the presence of total chlorine and total bromine. As the total chlorine and bromine levels increase, the color deepens to gold then brown.


The condition of having elevated cyanuric acid levels in pool water, generally acknowledged as concentrations above 90 ppm, that may diminish the sanitizing capabilities of chlorine, therefore making chlorine less effective.


The process of changing or destroying contaminants by increasing the number of oxygen atoms or reducing the number of electrons in the contaminant.


To combine with oxygen; See Oxidation.



The negative logarithm of the hydrogen ion concentration of a water solution; a scale ranging from 0.0 to 14.0, values below 7.0 are considered acidic, above 7.0 are basic or alkaline. The pH value of 7.0 is neutral, neither acid or base. Because the pH scale is logarithmic, each whole number increment represents a multiplier of 10. A pH of 8.0 is 10x more basic than 7.0 and a pH of 9.0 is 10x more basic than 8.0 or 100x more basic than 7.0.

pH Minus

The hth® brand name for sodium bisulfate, also called dry acid and used to reduce pH and/or total alkalinity of pool water.

pH Plus®

The hth® brand name for sodium carbonate, also called soda ash and used to raise pH.  It will also raise total alkalinity.

Phenol Red

A dye reagent used to test pH. Phenol Red is accurate only in the pH range of 6.8 to 8.4. The color is yellow below 6.8 and purple above 8.4 with varying shades of red between.

Pool Stabilizer

See Cyanuric Acid.

Potassium Monopersulfate

A chemical compound designed to oxidize organic contaminants in a pool or spa. These oxidizers have no capabilities to kill bacteria or algae. Also referred to as potassium peroxymonosulfate.

PPM (Parts per Million)

The accepted measurement of chemical concentration in swimming pool water. Equivalent to one gram per one million grams of water. Also expressed as mg/L.


A solid material which is forced out of a solution by some chemical reaction, which may settle out or remain as a haze in suspension (turbidity).


Pounds per square inch.


Quats (Quaternaries)

A type of algaecide synthesized with ammonia compounds.



To kill bacteria and other disease causing organisms.


A chemical used to kill bacteria and other disease causing organisms.

Saturation Index

See Langelier Saturation Index.


A gritty substance made up of calcium carbonate that can form on pool surfaces, plumbing and other pool components when the pH, total alkalinity, and calcium hardness are too high, or the Saturation Index is above +0.3.A gritty substance made up of calcium carbonate that can form on pool surfaces, plumbing and other pool components when the pH, total alkalinity, and calcium hardness are too high, or the Saturation Index is above +0.3.

Shock/Shock Treatment

A highly concentrated dose of chlorine that raises FAC in order to remove chloramines, control algae and kill bacteria and germs.


A device installed in or close to the pool wall for the purpose of cleaning the water's surface.

Sodium Bicarbonate (Baking Soda)

A dry chemical compound used in swimming pool water to raise total alkalinity.

Sodium Bisulfate (Dry Acid)

A dry chemical compound used in swimming pool water to reduce pH and total alkalinity. This is the ingredient in hth® pH Minus.

Sodium Carbonate (Soda Ash)

A dry chemical compound used in swimming pool water to raise pH. This is the ingredient in hth® pH Plus®.

Sodium Dichloroisocyanurate (Dichlor)

A white, fast-dissolving granular organic chlorine sanitizer, 55% to 59% available chlorine with pH of 6.0, can be used in pools and spas.

Sodium Hypochlorite (Liquid Chlorine, Bleach)

An unstabilized inorganic liquid chlorine solution, contains 10% to 15% available chlorine with a pH of 13.0; Bleach can break down quickly in sunlight.

Soft Water

A condition where the calcium and magnesium levels in the water are very low. Such water tends to be corrosive especially to surfaces in a pool or spa containing calcium.


Tiny asexual  reproductive cells, similar to seeds, for many forms of plant life, including algae and some bacteria.Tiny asexual  reproductive cells, similar to seeds, for many forms of plant life, including algae and some bacteria.

Stabilized Chlorine

See Isocyanurates.

Stabilized Pool

A pool treated with cyanuric acid to reduce loss of chlorine due to sunlight.


A white dry chemical compound that reduces the loss of chlorine to UV rays; recommended range 20-25 ppm is ideal, however, 20-50 ppm is acceptable. Do not exceed 90 ppm. High levels of Cyanuric Acid (CYA) or Stabilizer may diminish the sanitizing capabilities of chlorine, therefore making chlorine less effective.

Super Chlorination

The addition of much larger than normal dosages of a chlorinating compound for the purpose of destroying algae, slime, microorganisms or other contaminants in the water.


Trichloroisocyanuric acid (Trichlor)

A slow dissolving organic chlorine compound used to sanitize pools; most often compressed into tablets or sticks for use in feeders or skimmers. Typically has 80 - 90% available chlorine with a pH of 2.9. Trichlor is not recommended for use in a spa. This is the active ingredient in products like hth® Dual Action 3" Chlorinating Tablets,  hth® Super 3" Chlorinating Tablets, and hth® Ultra 3” Chlorinating Tablets.

Test Kit

A collection of reagents and chambers organized for analysis and determination of levels of certain critical factors in pool or spa water.

Total Alkalinity (TA)

The amount of carbonates, bicarbonates, and hydroxides in the pool. A high total alkalinity causes pH to resist adjustment to the desired range. A low total alkalinity makes it difficult to maintain pH within the desired range.

Total Chlorine (TC)

The sum of both free available chlorine (FAC) and combined chlorine (CC).

Total Dissolved Solids (T.D.S.)

The sum of all materials (solids) dissolved in the water. High TDS can interfere with sanitization and cause the water to be hazy and dull in appearance. TDS can be reduced only by dilution.


Water Hardness

See Hardness.


A physical and chemical treatment procedure used to close pools for the winter or off season and varies according to climate.