Whether you're new to the pool service industry or a seasoned veteran, understanding the terminology is essential for professional success. This comprehensive glossary covers over 100 terms you'll encounter daily, from water chemistry fundamentals to industry slang that only true pool pros know.
What This Glossary Covers
- Water chemistry terms and chemical names
- Equipment and pool components
- Testing methods and procedures
- Algae types and treatment terminology
- Industry jargon and slang
- Certifications and organizations
Water Chemistry Terms
Free Chlorine (FC)
The amount of chlorine available in the pool water to actively sanitize and disinfect. This is the chlorine that's doing the work, killing bacteria, algae, and other contaminants. Measured in parts per million (ppm), with ideal levels typically between 1-3 ppm for residential pools.
Learn moreTotal Chlorine (TC)
The sum of free chlorine plus combined chlorine in the water. When total chlorine is higher than free chlorine, you have combined chlorine present, indicating it's time to shock the pool.
Learn moreCombined Chlorine (CC)
The difference between total chlorine and free chlorine. Combined chlorine forms when free chlorine bonds with nitrogen compounds from sweat, urine, and other organic contaminants. This is what causes the "chlorine smell" and eye irritation at pools. Should be kept below 0.2 ppm.
Learn moreHypochlorous Acid (HOCl)
The active, killing form of chlorine in pool water. When chlorine is added to water, it forms hypochlorous acid, which is the actual sanitizer. Its effectiveness is pH-dependent, with more HOCl present at lower pH levels. At pH 7.2, chlorine is about 66% effective; at 7.5, it's 50% effective.
Learn moreHypochlorite Ion (OCl-)
The weaker form of chlorine that forms at higher pH levels. While it still sanitizes, it's less effective than hypochlorous acid. This is why maintaining proper pH is crucial for chlorine effectiveness.
pH
A measure of how acidic or basic the pool water is, on a scale of 0-14. Pool water should be maintained between 7.2-7.8, with 7.4-7.6 being ideal. Low pH is corrosive; high pH reduces chlorine effectiveness and can cause scaling.
Learn moreTotal Alkalinity (TA)
A measure of dissolved alkali (carbonates, bicarbonates, hydroxides, and cyanurates) in the water. Acts as a pH buffer, preventing rapid pH fluctuations. Ideal range is 80-120 ppm. Keep it lower (around 80) in summer and higher (around 120) in winter.
Learn moreCarbonate Alkalinity
The portion of total alkalinity that comes from carbonates and bicarbonates only, excluding cyanuric acid's contribution. Calculated by subtracting one-third of your CYA reading from your total alkalinity. This is the more accurate number to use for water balance calculations, especially for LSI calculations.
Learn moreCalcium Hardness (CH)
The amount of dissolved calcium in the water, measured in ppm. Ideal range is 200-400 ppm. Too low causes water to be corrosive and can etch plaster; too high causes scale buildup on surfaces and equipment.
Learn moreCyanuric Acid (CYA)
A chemical that protects chlorine from UV degradation by forming chlorinated cyanurates. Without CYA, chlorine can be destroyed by sunlight in minutes. Ideal range is 30-50 ppm. The industry caps it at 90 ppm because high levels slow down chlorine's sanitizing speed. CYA does not evaporate and can only be reduced by replacing water.
Learn moreORP (Oxidation Reduction Potential)
A measurement of the water's sanitizing ability in millivolts (mV). Higher ORP indicates greater oxidizing power. Commercial pools typically require a minimum of 650 mV. CYA suppresses ORP readings because sensors measure HOCl activity, not total chlorine. This is why ORP can read higher in the morning (when UV isn't strengthening the CYA-chlorine bond) and lower during the day.
Learn moreLSI (Langelier Saturation Index)
A calculation that predicts whether pool water will be corrosive, balanced, or scale-forming based on pH, temperature, calcium hardness, total alkalinity, and TDS. A balanced LSI is between -0.3 and +0.3. Negative values indicate corrosive water; positive values indicate scaling potential. Learn more in our LSI Calculator Guide.
Learn moreTDS (Total Dissolved Solids)
The total amount of dissolved substances in pool water, including minerals, salts, and organic matter. High TDS (above 1500-2000 ppm for non-saltwater pools) can reduce chlorine effectiveness and cause water to feel "heavy." Saltwater pools naturally have higher TDS (around 3000-4000 ppm).
Learn moreBreakpoint Chlorination
The process of adding enough chlorine to eliminate all combined chlorine (chloramines) from the water. When chlorine is added incrementally, it first bonds with nitrogen compounds to form mono-, di-, and trichloramines. At breakpoint, trichloramine gases off, eliminating the chloramine smell. Contrary to outdated "10x rule" advice, you only need slightly more chlorine than the combined chlorine level to achieve breakpoint.
Learn moreSuperchlorination
Adding a large dose of chlorine to the pool to achieve breakpoint chlorination, kill algae, or oxidize contaminants. The term "shock" technically refers to the action (verb), not the product (noun), though manufacturers often label products as "shock."
Learn moreCT Value
Concentration (of chlorine) multiplied by Time. Used to determine how long a certain chlorine level must be maintained to kill specific pathogens. For example, killing Cryptosporidium requires extremely high CT values, especially in pools with cyanuric acid.
Learn moreChemicals & Products
Cal Hypo
A granular chlorine product, typically 65-78% available chlorine. Raises calcium hardness and pH. Unstabilized, so it doesn't add CYA to the water. Often used for shocking pools. Formula: Ca(OCl)₂
Learn moreSodium Hypochlorite
Liquid chlorine, typically 10-12.5% concentration (pool grade) vs. 5-6% for household bleach. Raises pH when added. Unstabilized. Degrades over time, especially in heat and sunlight. Formula: NaOCl
Learn moreTrichlor
Stabilized chlorine tablets or granules, 90% available chlorine. Contains cyanuric acid, so it adds stabilizer to the water. Lowers pH and alkalinity. One pound of trichlor adds approximately 10 ppm chlorine and 6 ppm CYA per 10,000 gallons. Formula: C₃Cl₃N₃O₃
Learn moreDichlor
Stabilized granular chlorine, 56-62% available chlorine. Contains cyanuric acid. pH neutral. Often used for spas and hot tubs because it dissolves quickly. Formula: C₃Cl₂N₃NaO₃
Learn moreMuriatic Acid
A strong acid used to lower pH and total alkalinity. One gallon lowers alkalinity approximately 30-40 ppm per 10,000 gallons. Handle with care; it's corrosive and produces harmful fumes. Never pour directly on pool surfaces.
Learn moreBicarb
Used to raise total alkalinity. One pound raises alkalinity approximately 10 ppm per 10,000 gallons. Also slightly raises pH. Pool pros buy it in 40-pound bags.
Learn moreSoda Ash
Used to raise pH. More effective at raising pH than bicarb, with less impact on total alkalinity. Often used when you need to raise pH without significantly affecting TA.
Learn moreBorates
Added to pool water at 30-50 ppm to help stabilize pH and inhibit algae growth. Acts as an algistat (prevents algae) rather than an algaecide (kills algae). Maximum safe level is 50 ppm due to potential health concerns for pets who drink pool water.
Learn moreNon-Chlorine Shock
An oxidizer that doesn't contain chlorine. Despite the name, it does NOT eliminate chloramines because it's not strong enough to break the nitrogen-chloride bond. Useful for oxidizing organic contaminants and as a preventative maintenance product.
Learn morePhosphate Remover
A product that removes phosphates from pool water. Phosphates are nutrients that algae need to grow. When added, the product binds to phosphates and causes them to precipitate out, requiring filtration or vacuuming to waste.
Learn moreAlgae & Treatment
Green Algae
The most common type of pool algae. Floats freely in the water, causing a green tint. Relatively easy to treat with proper chlorination. Often appears first on walls and steps before spreading throughout the pool.
Learn moreYellow Algae
A stubborn form of algae that clings to pool walls and surfaces, often in shaded areas. Appears yellowish-brown and brushes off easily but returns quickly. More chlorine-resistant than green algae; often requires sodium bromide treatment.
Learn moreBlack Algae
The most difficult algae to eliminate. Forms dark blue-green or black spots that root into porous surfaces like plaster. Has a protective outer layer that must be brushed through before treatment can penetrate. Can even grow above the waterline in grout lines if conditions are right.
Learn morePink Slime
Not actually algae but a bacteria that appears as pink or reddish slime, typically in PVC pipes, skimmers, and shaded areas. Often accompanied by white water mold. Requires higher chlorine levels and thorough brushing to eliminate.
Learn moreWhite Water Mold
A fungal organism that appears as white, tissue-like sheets in the water or on surfaces. Like pink slime, it's a biofilm issue rather than true algae. Often indicates inadequate circulation or sanitization.
Learn moreAlgaecide
A chemical product that kills algae. Various types include quaternary ammonium compounds, polyquats, copper-based, and silver-based products. Each type has different effectiveness against different algae types.
Learn moreAlgistat
A chemical that prevents algae growth but doesn't necessarily kill existing algae. Borates are an example of an algistat. Prevention-focused rather than treatment-focused.
Learn moreQuaternary Ammonium Algaecide
A type of algaecide that can cause foaming if overdosed. Does not contain metals, so no staining risk. Commonly found in lower-cost algaecides. Not recommended for spas or fountains due to foaming tendency.
Learn morePolyquat Algaecide
A non-foaming quaternary algaecide. Thick, syrupy liquid. Works well on mustard algae. Does not contain metals. Often marketed as providing 60-day protection.
Learn moreCopper Algaecide
Highly effective algaecide that works by lysing (rupturing) algae cell membranes. Available in liquid and solid forms. Risk: can cause blue/purple copper cyanurate staining on plaster if copper levels get too high or water chemistry is imbalanced. Always test copper levels when using.
Learn moreSodium Bromide
A treatment for yellow/mustard algae. When added to the pool, chlorine converts it to bromine (hypobromous acid). The presence of bromide bypasses cyanuric acid's protection, making chlorine more immediately active. Requires adding extra chlorine because the reaction consumes it.
Learn moreAmmonium Sulfate
An algae treatment that requires specific conditions to work: pH of 8.0+, then superchlorination after treatment to remove the ammonia. Often misused when pool pros don't raise pH first or don't add enough chlorine afterward.
Learn moreFlock
A chemical (often aluminum sulfate) that causes suspended particles to clump together and sink to the pool floor. Requires vacuuming to waste afterward. Used for severe cloudiness or to lower cyanuric acid levels (at higher doses). The alum method for CYA reduction uses 8.33 lbs per 10,000 gallons.
Learn moreBiofilm
A protective layer that bacteria and other microorganisms form on surfaces. Acts as a shield against sanitizers. Must be physically disrupted (through brushing) for chemicals to be effective. Often found in pipes, on pool walls, and in equipment.
Learn moreEquipment & Pool Components
Skimmer
The opening on the side of the pool that draws in surface water. Contains a basket to catch debris before it reaches the pump. Connected to the suction side of the circulation system.
Learn morePump Basket
A basket located at the pump that catches debris that passes through the skimmer basket. Protects the pump impeller from damage. Should be cleaned regularly.
Learn moreImpeller
The rotating component inside the pump that moves water through the system. Can become clogged with debris or calcium buildup. A damaged or clogged impeller reduces flow rate.
Learn moreCartridge Filter
A filter type that uses pleated fabric cartridges to trap debris. No backwashing required; cartridges are removed and cleaned or replaced. Provides finer filtration than sand (10-15 microns).
Learn moreSand Filter
A filter type that uses sand as the filtration medium. Cleaned by backwashing (reversing water flow). Filters particles down to about 20-40 microns. Sand should be replaced every 5-7 years.
Learn moreDE Filter
The finest filtration available (2-5 microns). Uses a powder made from fossilized diatoms coated on grids. Must be backwashed and recharged with fresh DE powder. Produces the clearest water but requires more maintenance.
Learn moreBackwash
The process of reversing water flow through a filter to flush out trapped debris. Used on sand and DE filters. Water exits through the waste line rather than returning to the pool.
Learn moreMultiport Valve
A valve on sand and DE filters with multiple positions: Filter, Backwash, Rinse, Waste, Recirculate, and Closed. Never move the handle with the pump running to avoid water hammer.
Learn moreRecirculate
A valve position that bypasses the filter, allowing water to circulate without filtration. Used when adding certain chemicals (like alum) that shouldn't enter the filter media.
Learn moreVacuum to Waste
A method of vacuuming where debris and water are sent directly to the waste line, bypassing the filter. Used for heavy debris, algae, or flock that would clog the filter. Lowers water level, so monitor while vacuuming.
Learn moreSalt Cell
A device in saltwater pools that uses electrolysis to convert dissolved salt into chlorine. Cells have a limited lifespan (3-7 years) and require periodic cleaning to remove calcium scale. For the last time: a saltwater pool IS a chlorine pool.
Learn moreHeat Exchanger
The component in a pool heater where heat transfers to the pool water. Often made of copper. Susceptible to corrosion from low pH/alkalinity water. Can develop copper leaks that show up as elevated copper levels in the water.
Learn moreVSP
A pump that can run at different speeds. More energy efficient than single-speed pumps because it can run at lower speeds for longer periods. Required by DOE regulations for most new installations.
Learn moreAir Release Valve
A valve on top of the filter that allows air to escape when priming the system. Open when starting the pump; close when water (not air) starts coming out.
Learn moreReturn Jets
The fittings in the pool wall where filtered water returns to the pool. Adjustable eyeballs direct water flow. Should be aimed to promote circulation and surface skimming.
Learn moreMain Drain
The drain at the bottom of the pool that helps with circulation and lowering water level. Federal law (Virginia Graeme Baker Act) requires compliant covers or dual drains to prevent suction entrapment.
Learn morePlaster
The interior finish of a concrete pool, applied over gunite. Made from a mixture of cement, sand, and marble dust or other aggregite. Typical lifespan is 8-15 years. Susceptible to etching from low pH and staining from metals.
Learn moreGunite
A mixture of concrete, sand, and water sprayed onto the pool shell framework. Forms the structural shell of the pool. Plaster is applied on top of gunite as the visible finish.
Learn moreTile Line
The decorative tile band at the waterline of many pools. Helps mark water level and is easier to clean than plaster. Can develop scum line buildup and calcium deposits.
Learn moreScum Line
The buildup of oils, lotions, and organic matter at the waterline. Can harbor bacteria and algae if not cleaned regularly. Health departments can cite pools for excessive scum lines.
Learn moreCoping
The cap or edging around the top of the pool wall where the pool meets the deck. Can be precast concrete, natural stone, brick, or other materials.
Learn moreTesting Methods
DPD Test
A colorimetric method for testing chlorine levels. Uses three reagents: DPD-1 and DPD-2 test free chlorine; adding DPD-3 tests total chlorine. The difference indicates combined chlorine. Most common method in the field.
Learn moreFAS-DPD Test
A titration test for chlorine using a powder reagent. More accurate than standard DPD, especially at high chlorine levels (above 10 ppm). Counts drops to a color change rather than matching colors. Required for confirming chlorine levels during fecal incident response.
Learn morePhenol Red
The reagent used to test pH. Works in the range of 6.8-8.4. At chlorine levels above 15 ppm, chlorine bonds with phenol red to create chlorophenol red, which reads purple/blue regardless of actual pH. This is why you can't trust pH tests during shocking.
Learn moreDilution Test
A method for testing chemical levels that exceed the test kit's range. Mix pool water 50/50 with distilled or bottled water, then double the result. Often used for cyanuric acid above 100 ppm. The reason regulators cap CYA at 90 ppm is partly because they don't trust operators to do this correctly.
Learn moreDot Disappears Test
The visual method for testing cyanuric acid using a Taylor kit. Fill the tube until you can no longer see the black dot on the bottom. Standard test reads 30-100 ppm; higher levels require dilution.
Learn moreAcid Demand Test
A titration test that determines exactly how much acid is needed to lower pH to a target level. More accurate than using general dosing charts, which are rules of thumb.
Learn morePhotometer
An electronic device that measures chemical levels by analyzing light absorption through a water sample. More accurate and consistent than visual color matching. The Spin Touch by LaMotte is a popular example that tests 10+ parameters in 60 seconds.
Learn moreTaylor Test Kit
The industry-standard brand of water testing kits. The K-2006 is the most comprehensive kit for pool professionals. Named after Taylor Technologies.
Learn moreIndustry Slang & Jargon
Splash and Dash SLANG
Derogatory term for rushing through a pool service without doing thorough work. Just dumping chemicals and moving on without proper testing, brushing, or cleaning. The opposite of professional service.
Learn moreGreen to Clean SLANG
The process of restoring a pool from a severe algae bloom (green, murky water) to clear, swimmable condition. Often takes multiple visits and significant chemical treatment. Can also refer to a specific product (ammonium sulfate).
Learn moreChlorine Lock MYTH
A persistent myth that chlorine becomes "locked up" or stops working entirely at high CYA levels. In reality, high CYA slows chlorine's sanitizing speed but doesn't completely stop it. What people call chlorine lock is usually excessive CYA combined with high organic demand or biofilm issues.
Learn moreRoute SLANG
A pool technician's scheduled list of pools to service, typically organized by day and geographic area. "I've got 20 pools on my Thursday route."
Learn moreAcid Abuse SLANG
The common tendency to overuse muriatic acid, often because technicians underestimate its strength. Can cause plaster etching, equipment corrosion, and unsafe pH drops. Proper dosing and acid demand testing prevents this.
Learn moreFlash Testing SLANG
Old-school method of just adding one drop of reagent to see if any chlorine is present (color appears = has chlorine) rather than doing a proper measurement. Not accurate or professional.
Learn moreFloater SLANG
Either (1) a solid stool in the pool (a "normal" fecal incident vs. diarrhea), or (2) a floating chlorine dispenser that slowly releases tablets.
Learn moreGallonage SLANG
The volume of a pool in gallons. Technically correct since we're measuring what the pool holds, not just the space (which would be "area" or "volume"). "What's the gallonage on this pool?"
Learn moreTab Feeder SLANG
An inline or offline chlorinator that holds trichlor tablets. Also called a chlorine feeder or tablet feeder.
Learn moreProm Night Blue SLANG
Describes the deep blue/purple staining that occurs when copper cyanurate forms on plaster. Refers to the color being "bluer than your balls on prom night." Difficult to remove once formed.
Learn moreChase the Chunks SLANG
Scooping solid waste out of a pool during a fecal incident response. Also called "chunk the corn" or simply "scoop the poop."
Learn moreAndrea Perfect SLANG
Doing a thorough, high-quality job on every pool rather than rushing. Coined on the Talking Pools Podcast referring to co-host Andrea Nannini's standards. The opposite of splash and dash.
Learn moreCertifications & Organizations
CPO
A certification from the Pool & Hot Tub Alliance (PHTA) demonstrating competency in pool and spa operation. Required for commercial pool operators in many jurisdictions. Valid for 5 years. See our complete CPO certification guide.
Learn moreIPSSA
A trade association for pool service professionals. Offers training, networking, and industry resources. Known for their local chapter meetings and "sick route" coverage programs.
Learn morePHTA
The national trade association for the pool and spa industry. Develops industry standards (ANSI/APSP), runs the CPO certification program, and hosts the International Pool Spa Patio Expo.
Learn moreCDC
The federal agency that issues guidelines for recreational water illness prevention, including fecal incident response protocols. Their Crypto (Cryptosporidium) protocols require elevated chlorine levels and extended contact times, especially in pools with cyanuric acid.
Learn moreNSF
An independent organization that tests and certifies pool and spa products for safety and performance. Look for NSF certification on chemicals and equipment.
Learn moreVGB Act
Federal law enacted in 2007 requiring compliant drain covers and/or multiple suction outlets to prevent entrapment injuries and deaths. Named after a 7-year-old who died from suction entrapment.
Learn moreKeep Learning
Pool service terminology continues to evolve as new products and technologies enter the market. The best pool professionals stay current through continuing education, industry publications, and peer networking.