Pool & Hot Tub Water Chemistry Glossary
Plain-language definitions for every water chemistry term you will encounter when testing, balancing, and maintaining pool or hot tub water.
Terms A–Z
A
An algae bloom is the rapid, visible multiplication of algae in pool water, resulting in green, cloudy, or discolored water.
An algaecide is a chemical product that prevents algae growth or assists in killing established algae in pool water.
Aggressive (corrosive) water has an LSI below -0.
Ammonia in pool water is the primary nitrogen source for chloramine formation, introduced by bather waste including sweat and urine.
B
Breakpoint chlorination is the process of adding enough free chlorine to destroy all combined chlorine (chloramines) in pool water.
Backwashing is the process of reversing water flow through a sand or DE filter to flush accumulated debris out of the filter media to waste.
Bromine is a halogen sanitiser used in pool and spa water that forms hypobromous acid (HOBr) as its active disinfectant form.
BCDMH (1-bromo-3-chloro-5,5-dimethylhydantoin) is the most common active ingredient in bromine tablets for hot tubs and spas.
Black algae is a stubborn, deeply rooted algae species that forms black or dark blue-green spots on pool plaster surfaces.
Biofilm is a protective matrix of bacteria and organic material that establishes on pool and spa plumbing surfaces.
Bather load refers to the number and duration of swimmers in a pool, which affects chlorine consumption and organic contamination levels.
Bicarbonate alkalinity is the portion of total alkalinity contributed by bicarbonate ions (HCO3-), which is the dominant form in properly maintained pool water.
The breakpoint dose is the specific amount of free chlorine that must be added to destroy all combined chlorine in pool water.
C
Combined chlorine is the portion of total chlorine that has reacted with nitrogen compounds and is no longer an effective sanitiser.
Calcium hardness measures the concentration of dissolved calcium ions in pool water, expressed in ppm.
Cyanuric acid is a stabilizing compound that forms a reversible bond with free chlorine in pool water, protecting it from UV degradation.
A cartridge filter is a pool filter type that uses a pleated polyester element to capture particles as water passes from outside to inside the element.
Chloramines are compounds formed when free chlorine reacts with nitrogen-containing compounds in pool water.
Calcium hypochlorite is a granular chlorine compound used primarily as pool shock, with approximately 65–73% available chlorine.
Chlorine stabilizer is another name for cyanuric acid (CYA), used as a synonym especially in product marketing.
A pool clarifier is a coagulant product that causes fine suspended particles to clump together into larger particles that the filter can capture.
Chlorine demand is the total amount of chlorine consumed by all reactions in pool water before a free chlorine residual can be established.
Copper is a dissolved metal found in some pool water that causes blue-green staining when oxidised.
A pool closing chemical kit contains the chemicals needed to balance and protect pool water during winter dormancy.
Chemical compatibility refers to the safety of mixing or sequentially adding pool chemicals to water without dangerous reactions.
Carbon dioxide (CO2) in pool water forms carbonic acid, which influences pH and contributes to the natural downward pH drift in some systems.
Conductivity is a measure of water's ability to conduct electricity, which increases with the concentration of dissolved ions.
A chemical log is a written or digital record of every pool water test result and chemical addition, used for maintenance tracking and liability documentation.
A colorimetric test measures water chemistry by observing the colour change of a reagent added to a water sample.
Chemical storage refers to the safe storage of pool and spa chemicals to prevent degradation, accidents, and dangerous reactions.
D
Dichlor (sodium dichloroisocyanurate) is a fast-dissolving stabilised chlorine product that adds both free chlorine and cyanuric acid to pool water.
DPD (N,N-diethyl-p-phenylenediamine) is the standard reagent used in liquid test kits to measure free chlorine and total chlorine.
A DE (diatomaceous earth) filter uses diatomaceous earth powder coated on internal grids to filter pool water to 3–5 micron fineness.
Dilution refers to the reduction in chemical concentration in pool water caused by adding water (rain, refill, splash-out).
Dead algae is killed algae cells that remain suspended or settled in pool water after shock treatment, causing grey or white cloudiness.
E
Evaporation is the loss of water from a pool's surface as water molecules escape into the air, which concentrates dissolved chemicals and raises TDS.
F
Free chlorine is the concentration of active, unsaturated chlorine available in pool water to sanitise and disinfect.
Filter pressure is the water pressure reading on the filter pressure gauge, measured in psi (pounds per square inch).
A pool flocculant is a coagulating agent that causes all suspended particles to clump and sink to the pool floor for vacuuming to waste.
G
Green algae is the most common pool algae species, causing water to turn green or teal and surfaces to become slippery.
GPM is the unit measuring the flow rate of water through a pool pump or other equipment.
H
Hypochlorous acid (HOCl) is the active, bactericidal form of chlorine in pool water, responsible for killing pathogens.
I
Iron in pool water is a dissolved metal that can cause reddish-brown staining when oxidised by chlorine or shock treatment.
L
The Langelier Saturation Index is a calculated number that predicts whether pool water is in equilibrium with calcium carbonate, tending to corrode, or tending to deposit scale.
M
Muriatic acid (dilute hydrochloric acid) is the most commonly used chemical for lowering pool pH and total alkalinity.
Mustard algae is a yellow-brown pool algae species that brushes off easily but quickly returns, caused by a chlorine-resistant strain.
A metal sequestrant is a pool chemical that binds dissolved metals (iron, copper, manganese) to prevent them from staining pool surfaces.
A multiport valve is the control valve on a sand or DE filter that selects between filter, backwash, rinse, waste, and other operational modes.
The main drain is the fitting at the lowest point of a pool that allows the pool to be completely drained and provides a second suction point for the pump.
O
An oxidizer is a chemical that destroys organic contaminants in pool water through oxidation, without necessarily adding sanitiser residual.
ORP is the electrical potential of water measured in millivolts that indicates the overall oxidising capacity of the water.
Ozone (O3) is a powerful oxidiser used in pool systems to reduce organic contamination, chlorine demand, and chloramine formation.
OTO (orthotolidine) is an older test reagent that measures total chlorine by producing a yellow colour in chlorinated water.
A pool opening chemical kit is a package of chemicals assembled for spring pool opening, typically including shock, algaecide, and stain prevention.
P
pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 (most acidic) to 14 (most alkaline), with 7.
Pump turnover is the number of times the entire pool volume passes through the filter in a 24-hour period.
pH bounce is the condition where pool water pH swings rapidly and unpredictably, typically caused by insufficient total alkalinity.
Parts per million (ppm) is the unit used to express the concentration of dissolved substances in pool water, equivalent to milligrams per liter (mg/L).
Pool volume is the total capacity of a pool in gallons, used as the basis for all chemical dose calculations.
A pool pump is the mechanical device that circulates water through the filter, heater, and chlorinator, maintaining water movement and filtration.
Phosphates are nutrients present in pool water that can promote algae growth when free chlorine is inadequate.
A partial drain is the process of removing a portion of pool water and replacing it with fresh water to dilute accumulated dissolved compounds.
Pool turnover rate is the time required for the pool pump to circulate the entire pool volume through the filtration system once.
Pink slime is a pink or orange bacterial growth caused by Serratia marcescens or Methylobacterium species, commonly seen in hot tubs and pool fittings.
A pressure gauge on the pool filter measures water pressure inside the filter housing in psi, indicating filter cleanliness and flow resistance.
A pool volume reference is a pre-calculated table of approximate volumes for common pool dimensions, used for quick dose estimation.
A pool chemistry reference is a compiled table of target ranges, typical values, and adjustment guides for all major pool water parameters.
A pool shock schedule is the planned frequency and timing of pool shock treatments throughout the swimming season.
A pool maintenance schedule is a written plan covering daily, weekly, monthly, and seasonal maintenance tasks for pool chemistry and equipment.
Pool safety equipment includes the physical devices required to assist in pool emergency response: life ring, reaching pole, first aid kit, and emergency signage.
pH increaser is a pool product that raises water pH, typically sodium carbonate (soda ash) or sodium bicarbonate.
R
A reagent is a chemical used in a water test kit that reacts with the parameter being measured to produce a measurable colour change.
Return jets are the fittings in a pool wall through which filtered and treated water is returned to the pool.
S
A sanitizer is a chemical agent that reduces microbial contamination in pool water to safe levels for human contact.
Shock is the practice of adding a large dose of chlorine or oxidizer to pool water to reach breakpoint chlorination, kill algae, or reset water clarity.
A skimmer is a pool fitting built into the pool wall at the water surface that draws in surface water and floating debris for filtration.
A sand filter is a pool filter type that passes water through a bed of specially graded silica sand to capture particles.
Superchlorination is the practice of raising free chlorine to 10 ppm or higher to destroy chloramines, kill algae, or clear severely compromised water.
Sodium hypochlorite is liquid chlorine — a solution of sodium and hypochlorite ions used to sanitise and shock pool water.
Salt chlorination (salt water generation) is a system that produces free chlorine continuously by electrolyzing dissolved sodium chloride in pool water.
Sodium carbonate (soda ash) is used to raise pool water pH with minimal effect on total alkalinity.
Sodium bicarbonate (baking soda) is the standard chemical for raising total alkalinity in pool water.
Sodium bisulfate (dry acid) is a granular acid used to lower pool pH and total alkalinity as a safer alternative to liquid muriatic acid.
A scale inhibitor is a pool product that prevents calcium carbonate from precipitating onto pool surfaces, tile, and equipment.
Scaling water has an LSI above +0.
A saturation index is a calculated value predicting whether pool water will dissolve minerals (negative index) or deposit them as scale (positive index).
Shock dosage refers to the amount of shock product required to raise free chlorine to the target level for the pool volume and treatment goal.
Saturated pH (pHs) is the theoretical pH at which pool water would be exactly in equilibrium with calcium carbonate, used in LSI calculation.
T
Total alkalinity measures the water's ability to resist changes in pH, expressed as the total concentration of alkaline compounds (bicarbonate, carbonate, hydroxide) in ppm.
Total dissolved solids measures the total concentration of all dissolved substances in pool water, expressed in ppm.
Trichlor (trichloroisocyanuric acid) is a slow-dissolving stabilised chlorine tablet that delivers free chlorine and cyanuric acid simultaneously.
A test strip is a plastic strip with reagent-impregnated pads that produce colour changes when dipped in pool water, indicating chemical concentrations.
Turbidity is the cloudiness or haziness of pool water caused by suspended fine particles that scatter light.
Total hardness measures all divalent mineral ions in water (calcium and magnesium), while pool chemistry specifically uses only calcium hardness.
U
UV sanitization uses ultraviolet light to inactivate microorganisms in pool water as it passes through a UV chamber in the equipment line.
W
Winterization is the process of preparing a pool or hot tub for winter shutdown, including draining, blowing out plumbing lines, and protecting equipment from freeze damage.
Water temperature affects chlorine demand, chemical reaction rates, calcium solubility, and the Langelier Saturation Index.
A water balance checklist is a sequential list of tests and adjustments that ensures all pool chemistry parameters are brought into range in the correct order.
A water test log is a record sheet for documenting pool or hot tub water test results over time.