Pool Glossary: SLAM, Sanitizer, PSI, CYA & Common Pool Terms

If you have ever read pool advice and felt like everyone else was handed a secret manual, you are not alone. Pool care runs on shorthand — SLAM, CYA, FC, PSI, CC — and most of it never gets explained, because the people using it forgot they ever had to learn it too.

This glossary puts the terms you will actually run into in plain English: the ones on your test kit, inside the calculators, in equipment manuals, and in forum threads where everyone assumes you already know what “shock level for your CYA” means. They are grouped by what they relate to — water chemistry, safe swimming, equipment, filters, cleaning, pool surfaces, and the products on the store shelf — so you can jump straight to the part you need.

If you only learn a handful to start, make them FC (free chlorine), CYA (stabilizer), pH, TA (total alkalinity), PSI (filter pressure), and SLAM (how you clear algae). Those six explain most pool-care advice, most calculator inputs, and most troubleshooting steps. And one idea ties much of it together: on this site, the right free chlorine level is not a fixed number — it scales with your CYA.

Pool Chemistry Terms

These are the readings you test for and adjust. Keep them in range and most pool problems never get started.

FC: Free Chlorine

Free chlorine is the active chlorine available to sanitize the pool. It kills algae and germs. On this site, FC is not treated as a fixed 1-3 ppm number. The right FC level depends on your CYA level.

CC: Combined Chlorine

Combined chlorine is chlorine that has reacted with contaminants. High CC can mean the pool needs oxidation or a SLAM process. A common target is CC at or below 0.5 ppm.

TC: Total Chlorine

Total chlorine means free chlorine plus combined chlorine. If your test kit gives FC and TC, then CC is TC minus FC.

CYA: Cyanuric Acid / Stabilizer / Conditioner

CYA protects chlorine from sunlight. Outdoor pools usually need some CYA. Traditional liquid-chlorine pools often run around 30-50 ppm CYA, while salt water generator pools often run around 70-80 ppm. Higher CYA requires higher FC.

pH

pH tells you how acidic or basic the water is. Most pools feel and work best around 7.2-7.8, with 7.4-7.6 often used as the ideal working range.

TA: Total Alkalinity

Total alkalinity helps buffer pH so it does not swing too quickly. Traditional chlorine pools often use 80-120 ppm. Salt water generator pools often run lower, around 60-80 ppm, to help slow pH rise.

CH: Calcium Hardness

Calcium hardness measures dissolved calcium. Plaster/gunite pools need enough calcium to protect the surface. Vinyl and fiberglass pools usually need less calcium than plaster pools.

Salt

Salt is sodium chloride dissolved in the water. Salt water pools are still chlorine pools: the salt cell converts salt into chlorine. Most salt systems want roughly 2700-3400 ppm salt, but the exact range depends on the generator.

Sanitizer

A sanitizer is the chemical that kills germs and controls algae. In most residential pools, the sanitizer is chlorine. Salt water generators also sanitize with chlorine; they just make it from salt.

Shock

Shock can mean either a product or a process. On this site, the important idea is the process: temporarily raising FC high enough to oxidize contaminants or fight algae. Liquid chlorine is often the cleanest way to do that because it does not add CYA or calcium.

SLAM

SLAM means Shock Level And Maintain. It is a process for clearing algae or persistent contamination. You raise FC to the shock level for your CYA and keep it there until the pool passes the usual completion checks: clear water, low combined chlorine, and low overnight chlorine loss.

OCLT: Overnight Chlorine Loss Test

The OCLT checks whether something is still consuming chlorine. Test FC after sunset, then again before sunrise. If FC drops more than about 1 ppm overnight, algae or contamination may still be active.

ppm

ppm means parts per million. Pool test results for chlorine, CYA, alkalinity, calcium hardness, and salt are usually shown in ppm.

LSI / CSI

These are water-balance indexes used to estimate whether water may scale or etch surfaces. They matter most for plaster pools, salt cells, heaters, and high-calcium water.

Safe Swimming Terms

These few terms answer the only question that really matters before anyone gets in: is the water safe right now?

Safe to Swim

For normal swimming, check three things: FC is below 10 ppm, pH is in range, and the water is clear enough to see the bottom or main drain. Do not swim in cloudy water that hides the bottom.

Shock Level

Shock level is a temporary high FC target based on CYA, often around 40% of the CYA level. Shock level can be above the normal swimming range, so follow the safe-swimming checks before getting back in.

Water Clarity

Water clarity is a safety issue, not just a beauty issue. If you cannot clearly see the bottom or main drain, a swimmer in trouble may not be visible.

Equipment Terms

The hardware that moves and cleans your water — and the words manuals and forum threads use for each part.

Pump

The pump moves water through the pool system. It pulls water from the skimmer and drains, pushes it through the filter, and returns it to the pool.

Filter

The filter removes particles from the water. Common types are sand, cartridge, and DE filters.

PSI

PSI means pounds per square inch. On a pool filter gauge, PSI shows filter pressure. Rising PSI usually means the filter is getting dirty or flow is restricted.

Clean Pressure

Clean pressure is the PSI reading after the filter has been cleaned or backwashed. Many owners clean the filter when pressure rises about 20-25% above that baseline.

Skimmer

The skimmer pulls surface water into the filtration system and catches floating debris in a basket.

Main Drain

The main drain is the suction fitting at the pool floor. Some pools have one, some have two, and some above-ground pools have none.

Return Jet

Return jets send filtered water back into the pool. Aim them to improve circulation and reduce dead spots.

Weir Door

The weir door is the flap in the skimmer opening. It helps pull surface debris into the skimmer and keeps debris from floating back out.

Multiport Valve

A multiport valve controls water flow on many sand and DE filters. Common settings include filter, backwash, rinse, waste, recirculate, and closed.

Backwash

Backwashing reverses water flow through a sand or DE filter to flush out trapped debris. Cartridge filters are usually cleaned by removing and rinsing the cartridges, not backwashing.

Rinse

Rinse is a multiport setting used after backwashing. It settles the filter bed and sends dirty rinse water out the waste line before returning to normal filter mode.

Waste

Waste sends water out of the pool instead of through the filter. It is useful for vacuuming heavy debris or lowering water level.

Recirculate

Recirculate bypasses the filter and sends water back to the pool. It keeps water moving when the filter is not usable, but it does not remove debris.

Filter Types

Three filter styles do the same job in different ways, both in how finely they filter and in how you clean them.

Sand Filter

A sand filter uses special pool filter sand or glass media to trap debris. It is durable and easy to maintain, but it usually filters less finely than cartridge or DE.

Cartridge Filter

A cartridge filter uses pleated cartridges to trap debris. It does not backwash; you remove and rinse the cartridges when pressure rises or flow drops.

DE Filter

A DE filter uses diatomaceous earth powder on grids or fingers to filter very fine particles. It filters finely but requires more careful maintenance.

DE Powder

DE powder is a fine white filter powder used in DE filters. Do not add DE to a sand or cartridge filter unless the manufacturer specifically calls for it.

Cleaning And Maintenance Terms

The hands-on side of pool care: what you physically do to the water, and why each step matters.

Brushing

Brushing breaks up algae films, dust, and scale so the sanitizer and filter can do their jobs. Steps, corners, ladders, and shady walls need extra attention.

Vacuuming

Vacuuming removes debris from the pool floor. You can vacuum through the filter or to waste depending on the mess and your equipment.

Dead Spot

A dead spot is an area with poor circulation. Algae often starts in dead spots behind ladders, around steps, in corners, and under return patterns that do not mix well.

Turnover

Turnover means moving a volume of water equal to the pool volume through the system. It is a useful concept, but clear water depends on circulation, filtration, chemistry, and debris load, not turnover alone.

Prime

Prime means the pump is full of water and able to move water. A pump that loses prime may run dry, move little water, or show air in the pump basket.

Air Leak

An air leak lets air into the suction side of the system. Signs include bubbles in the pump basket, bubbles from return jets, or trouble keeping the pump primed.

Suction Side

The suction side is the plumbing before the pump, where water is pulled from the pool.

Pressure Side

The pressure side is the plumbing after the pump, where water is pushed through the filter, heater, chlorinator, and returns.

Surface And Pool-Type Terms

Your pool’s surface decides which chemistry rules matter most, so these terms come up constantly.

Plaster / Gunite / Concrete

These pools have cement-based surfaces. They are durable but sensitive to calcium balance, pH, and scaling or etching conditions.

Vinyl Liner

A vinyl liner is a flexible pool surface used in many above-ground and some inground pools. Avoid sharp tools, undissolved chemicals sitting on the liner, and harsh localized chemical contact.

Fiberglass

Fiberglass pools have a molded shell with a gelcoat surface. They usually need less calcium hardness than plaster pools but still need balanced water.

Waterline

The waterline is where the pool water meets the wall. Oils, sunscreen, scale, and debris often collect there.

Scale

Scale is mineral buildup, usually calcium carbonate. It often appears as white crust on tile, salt cells, heaters, and surfaces when pH, alkalinity, or calcium are high.

Etching

Etching is surface damage caused by aggressive water, often from low pH, low calcium, or poor balance in plaster pools.

Chemical Product Terms

What is actually inside the jugs and bags at the pool store, and what each one does once it hits the water.

Liquid Chlorine / Sodium Hypochlorite

Liquid chlorine is sodium hypochlorite. Pool-store liquid chlorine is often 10-12.5%; household bleach is usually weaker. It adds chlorine without adding CYA or calcium.

Cal-Hypo

Cal-hypo means calcium hypochlorite. It adds chlorine and calcium. It can be useful, but repeated use can raise calcium hardness.

Trichlor

Trichlor is a slow-dissolving stabilized chlorine, often sold as tablets or pucks. It adds chlorine, CYA, and acidity. Overuse can push CYA too high.

Dichlor

Dichlor is stabilized granular chlorine. It adds chlorine and CYA. It dissolves faster than trichlor but still raises CYA.

Muriatic Acid

Muriatic acid lowers pH and alkalinity. Add acid to water, never water to acid. Keep acid away from chlorine products.

Soda Ash / Sodium Carbonate

Soda ash raises pH and also raises alkalinity somewhat. It is different from baking soda.

Baking Soda / Sodium Bicarbonate

Baking soda mainly raises total alkalinity. It may nudge pH upward, but it is not the main chemical for large pH increases.

Calcium Chloride

Calcium chloride raises calcium hardness. It is most important for plaster pools and some fiberglass manufacturer requirements.

Algaecide

Algaecide helps prevent algae in some situations, but it is not a substitute for proper chlorine. For an active algae bloom, the SLAM process with liquid chlorine is usually the main treatment.

Clarifier

Clarifier helps tiny particles clump together so the filter can catch them. It is not a sanitizer and will not kill algae.

Flocculant / Floc

Floc makes small particles clump and sink so they can be vacuumed to waste. It can be useful in specific situations, but it is not a replacement for killing algae first.

Sequestrant

A sequestrant binds metals like iron or copper to reduce staining. It does not remove metals permanently; it holds them in solution and usually needs maintenance doses.

Common Acronyms

A quick-scan list of the abbreviations above, for when you just need the expansion.

• FC: free chlorine
• CC: combined chlorine
• TC: total chlorine
• CYA: cyanuric acid, also called stabilizer or conditioner
• TA: total alkalinity
• CH: calcium hardness
• SWG: salt water generator
• SLAM: Shock Level And Maintain
• OCLT: Overnight Chlorine Loss Test
• PSI: pounds per square inch, used for filter pressure
• GPM: gallons per minute, a flow-rate measurement
• ppm: parts per million
• DE: diatomaceous earth
• LSI/CSI: water-balance indexes for scaling or corrosion tendency

Bottom Line

You do not have to memorize all of this. Learn six terms first — FC, CYA, pH, TA, PSI, and SLAM — and you will follow most pool-care advice, calculator inputs, and troubleshooting steps. Bookmark this page and look up the rest whenever a new abbreviation trips you up.

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