Swimming Pool Terminologies, Abbreviations and Definitions

Posted by Michael Hervas on Aug 31, 2020

Are you confused by some of the swimming pool terminologies or abbreviations your pool guy or pool store is saying? Don't worry you're not the only one with this problem. But fret no more! Here are some of the terms, words, abbreviations and their meaning.

Baking Soda
Sodium Bicarbonate
Alkalinity Increaser

  • Common household baking soda is the same chemical pool stores sell for much more money to increase your total alkalinity.

Polyhexamethylene Biguanide

  • An alternative to chlorine. We do not recommend using PHMB unless you have a true chlorine allergy (which is extremely rare).


  • See sodium hypochorite, below, because that is what bleach is. It comes in 6% strength as Ultra bleach, 5.25% strength as regular bleach, and sometimes as 3% strength for some of the 'cheapie' bargain bleaches. You want to use plain, unscented bleach for your pool, not the scented or thickened kinds! One gallon will raise 10,000 gallons of water approximately the percentage of the bleach in FC (i.e, one gallon of 6% ultra bleach will raise 10,000 gallons approx. 6 ppm FC).


  • Borates can optionally be added to pool water to stabilize the pH, help prevent algae, reduce chlorine usage, and improve the water feel and appearance. When using borates, the recommended level is between 30 and 50 ppm.


  • An alternative to chlorine, most commonly used in hot tubs. Bromine cannot be effectively stabilized against sunlight, so is not normally used outdoors. Bromine is more stable than chlorine at high temperatures, so it is often used in hot tubs.

Calcium Hypoclorite
Cal Hypo

  • A slow dissolving unstabilized chlorine in powder or granular form. It has minimal impact on pH but it will raise calcium hardness with continued use. Suitable for normal sanitation or shocking and is the most common form of chlorine sold as "Shock". It can cloud your water if your TA is high and it is recommended to premix it in a bucket of water before adding to your pool. Relatively inexpensive. It is sold in different strengths with 48% and 65% being the most common. 2.5 oz of the 65% stuff will raise 1000 gallons 1 ppm FC.

Combined Chlorine

  • The stuff that smells like chlorine and burns your eyes that you get rid of by shocking your pool.

Calcium Hardness

  • This is the amount of calcium in your water and is most important if you have a plaster pool (marcite, exposed aggregate, quartz, etc.). It is not as important in vinyl pools. In fiberglass pools it helps prevent staining and cobalt spotting. Too high a calcium level can lead to scaling and cloudy water, too low can damage plaster surfaces. The recommended range is between 200-400 ppm for both plaster and fiberglass. It really is not an issue for vinyl pools unless it is very high. Anything above about 130 ppm is probably ok for vinyl and above 200 ppm for fiberglass. If there is a warranty on your pool or liner that specifies a calcium level then follow that to keep your warranty! It is tested with a titration (drop count) test that changes color from pink to blue.

Cyanuric Acid

  • Sometimes called stabilizer or conditioner, it helps protect the chlorine from 'burning off' in direct sunlight. Too little and most of your chlorine can be lost in about 30 minutes of direct sun. Too much and it interferes with your chlorine's ability to sanitizer and kill algae. Recommended range is 30-50 ppm unless you have a salt water chlorine generator, then the recommended range is somewhere between 50-100 ppm depending on the manufacturer of your unit.

Diatomaceous Earth

  • DE is made up of fossilized diatoms (porous, sponge like microscopic organism). It is used in DE filters to provide particularly fine filtering. Pool grade DE has been heat treated and is a mild carcinogen if the power is inhaled. DE is also available without heat treatment for use in gardening. Gardening DE is safer to handle but doesn't work at all well in filters.

Sodium dichloro-s-triazinetirone
Sodium dichloroisocyanurate

  • A stabilized, granular, fast-dissolving form of chlorine (chlorinated isocyanurate). Because of it's fast dissolving nature it is sometimes sold as 'shock' (not a good idea since it will raise CYA levels fairly quickly). It is mildly acidic and will lower pH and TA. Causes CYA to rise with continued use so it is not a good choice for those with cartridge filters or in climates that do not winterize pools and have an extended swim season. It will add .9 parts CYA for every 1 part chlorine it adds.

N, N-diethyl-paraphenylenediamine

  • Tests both FC and TC. You have to subtract the FC from the TC to get your CC. This test will turn different shades of red when chlorine is present and uses a color comparator that you match up the shade of red with. Many men have problems with this test because they are unable to differentiate the shades of red! This test can bleach out when chlorine levels go above 10 ppm and will turn colorless. This can lead you to believe that there is NO chlorine in your pool when it is actually very high. You can use OTO to verify if chlorine is present since it will not bleach out at high chlorine levels.

Ferrous ammonium sulfate-N, N-diethyl-paraphenylenediamine

  • A titration or drop count test for both FC and CC, to find your TC add your FC and CC readings. Measures chlorine levels up to at least 25 ppm with an accuracy of .2 ppm. This is the test you want! It does not suffer from the bleach out problem that the DPD test has and is very easy to read since the color change is from red to colorless.

Free Chlorine

  • The stuff that sanitizes your water.

Horse Power

  • HP provides a rough indication of the power of a pool pump. HP can be calculated in several different ways, so HP ratings can’t always be directly compared to each other. A more useful number is the SFHP, the rated HP times the rated SF (service factor). SFHP numbers can be compared to each other.


  • Ionizers are used to add metals to the water to help control algae. The most commonly used metals are copper, silver, and zinc. The costs, and risk of metal stains, normally make these systems a poor choice.

Lithium Hypochlorite

  • A fast dissolving unstabilized chlorine in powder form. It has minimal impact on pH or other water parameters. It is suitable for shocking or normal sanitation. It's convenience as a powder is offset by its extremely high cost!


  • "Minerals" is often used as a euphemism for metals added to the water to help control algae. The most commonly used metals are copper, silver, and zinc. The costs, and risk of metal stains, normally make these systems a poor choice.

Potassium Monopersulfate
Potassium Peroxymonosulfate

  • A non-chlorine oxidizer or 'shock'. It is acidic and will cause lower pH and TA. It adds sulfates to the water. Will not break down CC but helps prevent it from forming when used on a regular basis. It will interfere with TC and CC tests unless special reagents are used in testing since it WILL test as CC! A good choice to use in indoor pools that are not exposed to sunlight but really has no advantage in an outdoor pool. It is only an oxidizer and NOT a sanitizer and should not be used as such.

Material Safety Data Sheet

  • This is an information sheet that all manufacturers of pool chemicals must provide by law if there are any hazardous chemicals in a product. They are only required to list the hazardous ones and can say it's a proprietary mixture and a general description and not list the actual ingredients if they choose.


  • OTO will only test TC but it is a 'bulletproof' test that will not bleach out and will always tell you if there is chlorine in your pool. It will be different shades of yellow if chlorine is present and will turn orange to reddish or even brown at very high chlorine levels. It's a good test for a daily check to see if there is chlorine in your pool and is the test that is usually found in 2 way chlorine/pH testers.


  • Ozone is sometimes used as a supplement to chlorine to help oxidize organic debris in the pool. Ozone does not provide any residual sanitizer in the bulk pool water, so it is not suitable for use without a sanitizer.

Potential of Hydrogen

  • A measure of how acidic or basic (alkaline) a solution is. A pH of 7 is considered neutral, anything above 7 is basic and anything below is acidic. Pools are usually kept at a pH from 7.2 to 7.8. pH in pools is tested with the indicator phenol red. High sanitizer levels above about 5 ppm can interfere with this test and lead you to believe that your pH is very high when it is actually not! The better test kits will allow for this by adding a chlorine neutralizer to the indicator and calibrating their color comparator blocks to compensate for the added chlorine neutralizer so you can get accurate pH readings when your sanitizer levels are at 10-15 ppm.

Poly[oxyethylene(dimethyliminio)ethylene(dimethyliminio )ethylene dichloride]

  • The only algaecide we recommend using. Often sold in a 60% concentration as Algaecide 60 or something similar. Other kinds of algaecide have various problems, adding metals, foaming, bad smells, etc, depending on what they contain. Algaecide is most effective at preventing algae from getting started, and doesn't do very well against an active algae bloom.

Pool Salt

  • Sodium Chloride, used for Salt Water Chlorine Generators. It usually has a smaller crystal size than water softener solar salt and costs about twice as much. Ordinary water softener solar salt or pellets that have a purity of at least 99.5% and do not have any additives for cleaning water softeners or removing iron are fine to use in your pool and are what most manufacturers of SWGs recommend! The only advantage to pool salt is that it dissolves slightly faster. Do not use food grade salt in a pool under any circumstances. Food grade salt contains yellow prussiate of soda (sodium ferrocyanide) as an anti-caking ingredient and that can cause iron staining in your pool.

Parts per Million

  • A measure of the concentration of the chemicals in your pool.

Sodium Chloride

  • All swimming pools contain some salt, since there is some salt in most of the chemicals used swimming pools. Salt, at levels around 2000 ppm, can be used to improve the subjective feel of the water. Most SWGs require salt levels around 3000 ppm, though some require higher levels. Salt can contribute to accelerated corrosion of some materials, especially some of the softer kinds of natural stone and inferior grades of stainless steel. Ocean water typically has salt levels around 35,000 ppm, more than ten times the levels commonly used with a SWG.


  • Shocking is the process of raising your FC high enough to break down the CC that has formed, sometimes called breakpoint chlorination. "Shock" is a verb, not a noun! It is something you do to your pool, not a special product that you use. Any unstabilized chlorine is suitable to use in shocking.

Sodium Hypochorite

  • A liquid, unstabilized chlorine that is suitable for shocking or normal chlorination. It has minimal impact on pH and other water parameters. It comes in different strengths. The 12.5% strength is the most commonly used in pools and is easy to dose. Each ounce will raise 1000 gallons 1 ppm FC.

Total Alkalinity

  • TA is sometimes called carbonate hardness or kH. It has nothing to do with water hardness. This is a measure of the carbonates and bicarbonates in your water that help keep your pH from swinging up and down. It is tested with a titration (drop counting) test that has a distinct color change from green to red or from blue to light yellow, depending on your test kit.

Total Chlorine

  • TC is the total amount of both FC and CC in your pool, FC + CC = TC if your test kit uses FAS-DPD to test chlorine, TC - FC = CC if your test kit uses DPD to test chlorine.

Total Dissolved Solids

  • TDS measurements are not useful with swimming pools. Historically they were used as a proxy for CYA levels, before CYA testing became common. TDS measures total dissolved solids, when what you really need to know is which particular chemicals are present.

Total Hardness

  • TH is a measure of the levels of both calcium and magnesium in your water and is always going to be higher than your calcium hardness. It's not really a test that will be useful for a pool but many of the less expensive test kits and just about every test strip I have seen test for total hardness and not calcium hardness. If you have a drop-based test and it only uses 2 reagents instead of three it is most likely testing total hardness. This test also changes from pink to blue.

trichloroisocyanuric acid

  • A stabilized form of chlorine (chlorinated isocyanurate) usually sold in tablet or stick form for use in automatic chlorinators or in floating dispensers since it is very slow dissolving. It has a very low pH and will lower pH and TA. It is a good choice for new plaster pool startup chlorination. It will cause CYA levels to rise with continued use so it is not a good choice for those with cartridge filters or in climates that do not winterize pools and have an extended swim season. It will add .6 part CYA for every 1 part of chlorine it adds.


  • UV is sometimes used as a supplement to chlorine to help oxidize organic debris in the pool. UV does not provide any residual sanitizer in the bulk pool water, so it is not suitable for use without a sanitizer.