CHLORINE: The sanitizer/disinfectant/oxidizer that is most widely used by pool owners to kill any bacteria, living organisms, ammonia, or other contaminates that are present in water. For more specific information on chlorine, select from the following topics:
TYPES OF CHLORINE: There are (4) commonly used types of chlorine on the market:
CHLORINE CHART: Review the following chart for specific information on your chosen type of chlorine:
| Product | pH value | Available Chlorine | By-products | Form | Stability |
| Gas | 0.5-1.0 | 100% | 0 | Gas | Extremely unstable |
| Liquid | 13.0-14.0 | 12 - 15% | Salt | Liquid | Unstable |
| Granular | 6.8 | 63% | Cyanuric acid | Granular | Very stable |
| Tablets | 2.8-3.0 | 90% | Cyanuric acid | Tablet | Extremely stable |
CHLORINE GAS: The most elementary form of chlorine. Chlorine gas is produced in factories by administering actual electricity through a saline (salt) solution. The result is a mixture of a liquid and a gas. Although a liquid is present in the mixture, the final product is a gas. The advantages of chlorine gas are that it is fairly inexpensive, it utilizes 100% available chlorine to the pool owner, and because it is pure (100%) chlorine, it adds no by-products to the pool water. As great as that sounds, the one disadvantage greatly outweighs the numerous advantages - if misused, chlorine gas is deadly. Also, only those with both a textbook and a hands-on experience in Chemistry should be allowed to handle and administer chlorine gas. It is difficult to handle, making misuse easy.
LIQUID CHLORINE: Water is combined with the chlorine gas to produce liquid chlorine. The advantages of liquid chlorine are that it is inexpensive upfront and it is easy to use - simply read the instructions to determine if the liquid chlorine should either be first diluted in a bucket of water or broadcast straight from the bottle, then pour the liquid chlorine into the pool water, and it will work immediately. There are a few significant disadvantages to grade liquid chlorine as only marginally effective. The main disadvantage is that liquid chlorine is so unstable that by the time it is in your hands, there is only about 12%-15% available chlorine and this minimal amount can be further exhausted by both the sun's ultraviolet rays and a relatively short shelf life. Typically, liquid chlorine will lose its effectiveness in as little as 30 days. Another disadvantage is that although it seems inexpensive upfront, the lack of available chlorine causes pool owners to use more of the product to keep up with sanitizing, disinfecting, and oxidizing needs. Yet another disadvantage is that chlorine gas uses a saline solution, which elicits a salt by-product to the pool water.
GRANULAR CHLORINE: A powder form of chlorine. The advantages of Granular Chlorine are many. Granular Chlorine is fast dissolving so it should not bleach the pool walls or floor of any type of pool. Granular chlorine also possesses a 63% available chlorine, which is very effective at sanitizing, disinfecting, and oxidizing. Granular chlorine also possesses a built in Cyanuric Acid, which prevents the reduction of chlorine loss due to the sun. Since granular chlorine has high available chlorine and is fast dissolving, it can be used for regular pool chlorination or for shocking a pool, all without adding any by-products to the pool water. Granular chlorine will not cloud the water upon initial application. Granular chlorine also has pH value of 6.8, which is near the desired pH range (7.2 to 7.8). Finally, granular chlorine has a long shelf life. The only real disadvantage of granular chlorine is that it costs more than chlorine tablets. And since there is less available chlorine in granular chlorine than in chlorine tablets (63% for granular chlorine versus 90% for chlorine tablets), more will have to be used, which heightens the expense. If expense is not an issue, granular chlorine rates high as an effective sanitizer, disinfectant, and oxidizer.
CHLORINE TABLETS: Chlorine tablets come in two sizes: 1" tabs and 3" tabs (often referred to as pucks, as 3" tabs resemble the size and diameter of a hockey puck). Without a doubt, chlorine tablets are purchased by pool owners in the greatest volume and for good reason--Chlorine tablets are the best form of chlorine available on the market today. Chlorine tablets have a few distinct advantages. Chlorine tablets possess a 90% available chlorine, making it the strongest and most effective sanitizer, disinfectant, and oxidizer that is safely available to pool owners. Chlorine tablets also possess a built in Cyanuric Acid, which prevents the reduction of chlorine loss due to the sun. Chlorine tablets also cost less than granular chlorine. Another advantage of chlorine tablets is their long shelf life. The disadvantages of chlorine tablets are few and are easily corrected. Chlorine tablets have a pH value of 2.8 to 3.0, which is very acidic. Because it is so acidic, neglect of pH levels can cause chlorine tablets to assist in dissolving metallic materials (pool walls, floor, hand rails, ladders, light fixtures, and equipment), which may result in discolored water, stains, or the formation of scale. Although this can be seriously detrimental and costly if not treated upfront, an acidic problem can easily be rectified-test and adjust the pH often (as you should do anyway) and do not place chlorine tablets into the skimmer(s). A skimmer is a suction port where pool water is drawn out of the pool and sent through filtration. Therefore, the first item the water comes into contact with is the equipment and their metallic internal parts (primarily the internals of a pump and the copper internals of a heater), which can be dissolved by acidic water, resulting in premature equipment failure. Again, simply check the pH often and place the chlorine tablets in an automatic chlorine feeder. If you do not own an automatic chemical feeder, it really is a worthwhile investment to have your local pool professionals install one for you. Although an automatic chlorine feeder is best, a floater will work, but pool owners will constantly have to remove it from the skimmer, as that is where suction will inevitably draw it.
THE USE OF CYANURIC ACID WITH CHLORINE: As stated earlier, chlorine tablets and granular chlorine are the two most commonly used forms of chlorine. These two forms of chlorine are classified as Cyanurates, which simply means they contain Cyanuric Acid as an ingredient. Cyanuric Acid is a chemical that protects chlorine from being destroyed by the sunlight. Since chlorine is so susceptible to the sun's ultraviolet rays, Cyanuric Acid is a necessary ingredient.
Although chlorine tablets and granular chlorine contain Cyanuric Acid as an ingredient, the amount contained is merely a trace. Therefore, chlorine users may have to add additional Cyanuric Acid (which is typically packaged and sold as either "Conditioner" or "Stabilizer") periodically. Cyanuric Acid will definitely need to be added after a pool is drained, cleaned and refilled, as tap water contains almost no Cyanuric Acid. Cyanuric Acid may also need to be added once a pool is opened in the spring (this is obviously only for those pools that require a winterize). There may be other scenarios that require an addition of Cyanuric Acid ("Conditioner" or "Stabilizer").
Chlorine users should visit their pool professionals monthly to have them run a computer test for Cyanuric Acid. It should also be tested anytime you are struggling to obtain a sufficient chlorine reading. If there are just trace amounts of Cyanuric Acid in the pool water, then it does not matter how much chlorine (or chlorine-based shock) is added, because the majority of it will be destroyed by the sun.
The recommended ideal range for Cyanuric acid is 30-80 ppm, with a maximum, of 100 ppm. Poolmanual.com recommends an initial reading closer to 40 ppm as ideal, which is high enough to prevent the sun from destroying chlorine's effectiveness. Since chlorine tablets and granular chlorine are cyanurates, trace amounts of Cyanuric Acid are added to the water as this chlorine is continually added to the pool. Eventually then, the Cyanuric acid reading will eclipse 100 ppm (maximum that is accepted), which will cause the pool water to hold too strong of a chlorine reading. Although the main reason for skin and eye irritation is the presence of chloramines (and in effect, not enough chlorine), another reason for skin and eye irritation is too high of a chlorine reading.
There is no chemical to reduce Cyanuric Acid. It can only be reduced by either partially or completely draining the pool. It will take many years of adding chlorine tablets or granular chlorine to eclipse that 100-ppm reading. Remember, once drained and refilled, Cyanuric Acid ("Conditioner" or "Stabilizer") will need to be added again, because tap water contains almost no trace of Cyanuric Acid. And again remember, without Cyanuric Acid, your chlorine will be destroyed by the sun almost instantly.
NOTE: Cyanuric Acid is NOT used with bromine or any other alternative to chlorine. Cyanuric Acid is only used with chlorine.
DUTIES OF CHLORINE: Chlorine is a sanitizer, disinfectant, and oxidizer for water. By sanitizing, chlorine will kill all bacteria, living organisms, and other contaminates (dirt, debris, and algae spores) that are found in water. By disinfecting, chlorine will kill all potential disease-carrying capabilities of these bacteria, living organisms, and other contaminates. By oxidizing, chlorine will combine with, in order to kill, these invaders, primarily the ammonia that is found in water.
Chlorine, in any form will take a leadership role to sanitize, disinfect and oxidize pool water. But, you cannot just add chlorine from time to time and expect to have clean, clear, blue and sparkling pool water. Rather, the chlorine level must be continually monitored-almost daily-as it will fluctuate on a constant basis, primarily due to climate (temperature, humidity, sunlight, rain, wind, and evaporation) and bather load (the extent that you use your pool)-these are factors that will lead to additional bacteria, living organisms, ammonia, and other contaminates being constantly present in your pool water.
Bacteria, living organisms, ammonia, and other contaminates are present in any body of water, like your pool. If you do not believe this, do not add any chlorine to the water and see how quick it takes to transform your water from clean, clear, blue, and sparkling to cloudy, murky, and full of algae. We at poolmanual.com hope you understand this is a rhetorical dare. Rather, just let it be understood that these invaders are present in all water.
Chlorine will combine with these invaders in order to kill them. Once engaged in battle with these invaders, only a portion of the chlorine will remain available in the water (this portion is called Free Chlorine) while the remainder of the chlorine is occupied by a continued influx of these invaders. This Free Chlorine will not last long, as outside factors, such as climate (temperature, humidity, sunlight, rain, wind, and evaporation) and bather load (the extent that you use your pool) will add even more bacteria, living organisms, ammonia, and other contaminates to your pool water. This constant reintroduction of invaders will occupy your previously Free Chlorine, causing the Free Chlorine level to lessen.
This is a continuous cycle. Chlorine is constantly sanitizing, disinfecting, and oxidizing your pool water. Once the supply of chlorine is used up, reinforcements are needed. This is why chlorine levels must be tested and adjusted often: 3-4 times per week during normal conditions and normal use and almost daily during periods of scorching temperatures, unbearable humidity, and complete sunlight, as well as after heavy rains or fierce winds, and of course, after heavy bather loads; it also goes without mention that chlorine levels must be tested daily under conditions of poor water chemistry (cloudy or murky water, as well as the presence of algae). The influx of bacteria, living organisms, ammonia, and other contaminates is continuous-you had better test and adjust your chlorine often in order to meet and exceed these needs.
NOTE: Chlorine will combine with, but cannot successfully kill, ammonia by itself. For this reason, you must shock your pool on a regular basis.
NOTE: climate (which differs from region-to-region) and bather load (which differs from pool-to-pool) do determine how often you have to test and adjust your chemicals.
TERM: "CHLORINE DEMAND": The discussion on "Demand and Balance" focused on water being characterized as a solvent, saturating and precipitating any metallic materials that it came into contact with. Just like water places a demand on metallic materials within your pool until that demand has been met (by achieving and maintaining water chemistry/balance), invaders, such as bacteria, living organisms, ammonia, and other contaminates will place a demand on chlorine. Chlorine Demand is the amount of chlorine needed to kill all invaders in the water. After this demand is met, the amount of chlorine remaining in the water to continue meeting the demands of constant sanitizing, disinfecting, and oxidizing is known as called free chlorine. The Chlorine Demand-to-Free Chlorine ratio is a cyclical pattern.
In order to keep up with Chlorine Demand, keep your Free Chlorine level between 2.0 - 3.0 parts per million (ppm). Since these invaders are constantly introduced and reintroduced to your water, they will keep your chlorine level continuously occupied. Therefore, your Free Chlorine level will lessen-since it is "occupied" it is no longer "free." You will have to compensate for this diminishing Free Chlorine level by constantly adding chlorine to the pool in order to stay between 2.0 - 3.0 ppm on your chlorine test reading at all times. Therefore, test your chlorine level often: 3-4 times per week during normal conditions and normal use and almost daily during periods of scorching temperatures, unbearable humidity, and complete sunlight, as well as after heavy rains or fierce winds, and of course, after heavy bather loads; it also goes without mention that chlorine levels must be tested daily under conditions of poor water chemistry (cloudy or murky water, as well as the presence of algae).
It cannot be stressed enough that the war between chlorine and invaders of your pool water is an ongoing, and often, a daily battle. You must test and adjust your chlorine accordingly.
NOTE: climate (which differs from region-to-region) and bather load (which differs from pool-to-pool) do determine how often you have to test and adjust your chemicals
TERM: "FREE CHLORINE": Free Chlorine is the chlorine that is NOT currently combined with any invaders, such as bacteria, living organisms, ammonia, or other contaminates in your pool water. This is the chlorine that is not occupied and still available (measurable) in your pool water. But remember, invaders are constantly introduced and reintroduced to your pool water. A Free Chlorine reading of 2.0 parts per million (ppm) on one day may register under a 1.0 ppm reading the next day-especially if it is hot, humid, sunny, rainy, windy, or if the pool was heavily used. It is ideal to keep your Free Chlorine level between 2.0 - 3.0 ppm. Never allow your Free Chlorine reading to drop below 1.0 ppm. If a test shows Free Chlorine at 0.99 ppm or lower, you MUST add chlorine (either tablets or granular) to the pool water immediately; to be safe, you really should take immediate action when the Free Chlorine level gets closer to 1.5 ppm-its ALWAYS better to be safe that sorry.
Therefore, test your chlorine level often: 3-4 times per week during normal conditions and normal use and almost daily during periods of scorching temperatures, unbearable humidity, and complete sunlight, as well as after heavy rains or fierce winds, and of course, after heavy bather loads; it also goes without mention that chlorine levels must be tested daily under conditions of poor water chemistry (cloudy or murky water, as well as the presence of algae). It cannot be stressed enough that the war between chlorine and invaders of your pool water is an ongoing, and often, a daily battle. You must test and adjust your chlorine accordingly. You must have a strong Free Chlorine reading (between 2.0 - 3.0 ppm) on a continuous basis in order to keep up with the constant influx of invaders in your pool water.
NOTE: "Free Chlorine" is sometimes also called "Available Chlorine," "Free-Available Chlorine," "Usable Chlorine," "Remaining Chlorine," or "Chlorine Residual."
TERM: "CHLORAMINES": During the recent discussion on
Chlorine Demand, you learned that chlorine must be present in sufficient amounts
to kill all bacteria, living organisms, ammonia, and other contaminates.
Well, chlorine is very effective at killing bacteria, living organisms, and
other contaminates (which again are dirt, debris and algae spores), but when
ammonia is present, chlorine will combine with it, but cannot successfully
kill it, and the two combine to form "Chloramines." Chloramines will remain
in the water, and will register as chlorine, but are ineffective at sanitizing,
disinfecting, or oxidizing.
Ammonia is commonly present in pool water from either environmental factors, a build up of fertilizers that are carried by wind and dropped into pools, from swimmer wastes (perspiration, urine, saliva and body oils), or even from some suntan lotions. No matter how ammonia enters a pool, it will exist.
Chlorine Demand has been defined as the amount of chlorine needed to kill bacteria, living organisms, ammonia, and other contaminates. Free Chlorine has been defined as the amount of chlorine that is still available (measurable) in the water after Chlorine Demand has been met. Chloramines do not fall into the category of Free Chlorine, but they do register as chlorine in general. Therefore, Total Chlorine must now be defined as a reading of the combination of Free Chlorine and Chloramines.
So then, since Chloramines are ineffective and undesired forms of chlorine, how are they removed from the water? Easy-by shocking the pool on a consistent and regular basis. Even if you maintain a 2.0 - 3.0 ppm (ideal) chlorine reading at all times by merely adding chlorine tablets or granular chlorine to the pool, the presence of ammonia will lead to Chloramines, preventing the chlorine from being fully effective. The only way to combat Chloramines, and kill ammonia, is by shocking the pool.
NOTE: A common myth among swimmers is that too much chlorine is the only cause of eye and skin irritation. Although high chlorine is a definite cause, it is not the only reason-it is not even the leading cause. The presence of Chloramines, and in effect, too little Free Chlorine, is a primary cause for eye and skin irritation.
TERM: "TOTAL CHLORINE": the combined reading of Free Chlorine and Chloramines. Free Chlorine is desired in the pool water. Chloramines are undesired in the pool water. In order to kill the Chloramines, and at the same time, enhance the effectiveness of the Free Chlorine, you must shock your pool.
There are test kits to measure Total Chlorine. If you purchase this (DPD) test kit, it is a 2-part test. The first test measures Free Chlorine. The second test measures Total Chlorine. You can subtract the Free Chlorine FROM the Total Chlorine in order to determine the approximate amount of Chloramines in the pool water. If the difference is 1.0 ppm or higher, then the amount of Chloramines in the water is too high. You must then shock the pool to get rid of these ineffective and undesired Chloramines. Here is an example:
In order to determine when to add chlorine (either tablets or granular) itself to the pool, test for Free Chlorine. In order to determine when to shock the pool, test for Total Chlorine. Here is a simplified explanation:
You really do not even need to test for Total Chlorine. Just make sure to test Free Chlorine often (perhaps daily) in order to maintain a 2.0 - 3.0 ppm reading. Also, make sure to shock the pool on a regular basis. And, like all chemicals, shock more often when its hot, humid, sunny, rainy, or windy, as well as when bather loads are high, and of course, when poor overall water chemistry is evident (cloudy and murky water, and perhaps, the presence of algae).
NOTE: climate (which differs from region-to-region) and bather load (which differs from pool-to-pool) do determine how often you have to test and adjust your chemicals.
IDEAL READING FOR CHLORINE: You need to test the Free Chlorine reading often: 3-4 times per week during normal conditions and normal use and almost daily during periods of scorching temperatures, unbearable humidity, and complete sunlight, as well as after heavy rains or fierce winds, and of course, after heavy bather loads; it also goes without mention that chlorine levels must be tested daily under conditions of poor water chemistry (cloudy or murky water, and perhaps, the presence of algae). You need to maintain a consistent 2.0 - 3.0 ppm Free Chlorine reading. This is strong enough to meet Chlorine Demand. You do have some room for error. The industry standard is 1.0 ppm as an absolute minimum Free Chlorine reading; even though 1.0 ppm is acceptable, you are better off constantly maintaining that ideal 2.0 - 3.0 ppm reading. Testing for Total Chlorine is really not as necessary as testing Free Chlorine on a continuous basis. Rather, just make sure to shock the pool on a consistent and regular basis.
NOTE: climate (which differs from region-to-region) and bather load (which differs from pool-to-pool) do determine how often you have to test and adjust your chemicals.
WHY CHLORINE MUST BE MONITORED: Your chlorine is involved in a constant battle with invaders, such as bacteria, living organisms, ammonia, and other contaminates (dirt, debris, and algae spores) that are present in water. Environmental factors, such as climate (temperature, humidity, sunlight, rain, wind, and evaporation) are constantly introducing and reintroducing more invaders to your pool water-it is a cyclical and constant battle; at times, it is a daily battle. Furthermore, as the temperature, humidity, and sunlight increase, so to will your bather load (the extent that you use your pool). It only makes sense that the hotter it is, the more often you will use your pool. You have to compensate for all of these factors. Your initial line of defense is understanding that water itself is never free of these invaders. From here, you need to engage in an implementation stage. The only way you can do this is by constantly testing and adjusting the chlorine level to assure that it is between 2.0 - 3.0 ppm, as well as by shocking regularly.
NOTE: climate (which differs from region-to-region) and bather load (which differs from pool-to-pool) do determine how often you have to test and adjust your chemicals.
TESTING FOR CHLORINE: There are two ways to test for chlorine: test strips or test kits.
The use of test strips is really self-explanatory. Read the instructions on the bottle for all the "how to" instructions. Some test strips will test both Free Chlorine and Total Chlorine, but these are expensive test strips. Most test strips will test for pH and Alkalinity, as well as the chlorine. Here are steps to using test strips:
The use of test kits is also popular. Read the instructions on the test kit for all the "how to" instructions. Some test kits will test both Free Chlorine and Total Chlorine. There are different types of chlorine test kits. Here is a discussion:
One reagent to test chlorine is called OTO. OTO is a colorimetric test that uses a reagent only. You simply:
The OTO test, which was once the standard test for chlorine, is no longer highly recommended. OTO can only measure Free Chlorine; OTO cannot distinguish between what is Free Chlorine and what are Chloramines. Also, OTO tests are prone to errors. The pH and the Alkalinity can affect the accuracy of an OTO test.
The most popular and accurate test now for chlorine is the DPD test. DPD will determine both Free Chlorine and Total Chlorine. DPD is a 2-part colorimetric test using either a liquid reagent or a tablet. Here are the steps:
In reagent form, DPD#1 is added to the sample to determine Free Chlorine. Then, DPD#2 is added to the same vial of the test block to determine Total Chlorine. If there is no color change in the sample from the DPD#1 reagent to the DPD#2 reagent, then no Chloramines are present in the water.
In tablet form, DPD#1 is added to determine Free Chlorine. Then, DPD#3 is added to the same vial of the test block to determine Total Chlorine. Like the reagents, if there is no color change in the sample from the DPD#1 tablet to the DPD#3 tablet, then no Chloramines are present in the water.
Aside from telling the entire story of chlorine, yet another advantage of the DPD test is that the reagents and tablets are produced with an ingredient that buffers pH, so tests are less likely to elicit false readings at varying pH levels. But, with DPD, here are words of caution:
NOTE: Make sure you read the instructions on the test kit that you purchase, regardless if it is an OTO test kit or a DPD test kit. They will tell you all the "how to" instructions:
WHEN TO ADD CHLORINE: The ideal range for Free Chlorine is 2.0 - 3.0 ppm. The industry minimum accepted Free Chlorine level is 1.0 ppm. It is the advice of poolmanual.com to add chlorine to the pool when a chlorine test shows the Free Chlorine level to be just lower than 2.0 ppm. Due to work schedules and errands, this is hard to do. But remember, you do have that safety net of 1.0 ppm as a minimum.
If you allow your Free Chlorine level to drop to 1.0 ppm before you add chlorine, you will get into trouble. Due to these same work schedules or errands, you may inadvertently neglect chlorine for a day or two, and the level will drop below that 1.0 minimum ppm-this is where poor water chemistry issues, such as cloudy or murky water, as well as the presence of algae, become very probable.
Therefore, test chlorine often: 3-4 times per week during normal conditions and normal use and almost daily during periods of scorching temperatures, unbearable humidity, and complete sunlight, as well as after heavy rains or fierce winds, and of course, after heavy bather loads; it also goes without mention that chlorine levels must be tested daily under conditions of poor water chemistry (cloudy or murky water, and perhaps, the presence of algae). Water chemistry, as far as chlorine is concerned, will be adequate as long as the Free Chlorine level is maintained between 2.0 - 3.0 ppm, and shocking is accomplished regularly.
NOTE: climate (which differs from region-to-region) and bather load (which differs from pool-to-pool) do determine how often you have to test and adjust your chemicals.
Without a doubt, the best way to add chlorine tablets to a pool is via an automatic chemical feeder (often called a chlorinator). Adding chlorine to an automatic chemical feeder is extremely easy. But, consult your local pool professionals for the following reason:
If you do not have an automatic chemical feeder, it really is a good investment to have your local pool professionals install one for you-it is an investment versus a service call. If you do not use an automatic chemical feeder, then place your chlorine tablets in a floater. Make sure to remove the floater from the skimmer; you will constantly have to do this, as suction will ultimately draw the floater to the skimmer. NEVER place the chlorine tablets directly in the skimmer basket(s).
If you use granular chlorine or liquid chlorine, add it directly to the pool. Read the instructions on the label to determine if you need to dilute the chlorine in a bucket of water first, or broadcast it directly from the container.
NOTE: Regardless if you use tablets, granular, or liquid, add the chlorine when a reading from your test shows the Free Chlorine level to be just lower than 2.0 ppm.
NOTE: There are few aboveground automatic chlorine feeders on the market. These automatic chlorine feeders are a worthwhile investment. If you decide against the automatic feeder, get a floater, but you will have to constantly remove it from the skimmer area, as this is where suction will inevitable draw it. NEVER place your chlorine, or nay chemical, in the skimmer.
HOW TO LOWER CHLORINE: 99% of the time, you will need to focus on maintaining and raising the chlorine levels in order to effectively continue to sanitize, disinfect, and oxidize the pool water. In very rare instances will you have to worry about lowering the level of chlorine. But, if chlorine levels get too high (above 5.5 ppm with a sufficient Cyanuric Acid reading of 30-95 ppm) on a consistent basis, then it is best to reduce (or altogether stop) adding chlorine to the pool:
It is recommended to allow the chlorine level to gradually drop by itself. But, if you need to use your pool soon and cannot wait for the chlorine level to gradually drop by itself, then go visit your local pool professionals and purchase Sodium Thiosulfate, which will lower chlorine levels. Since having to lower chlorine is fairly rare, few chemical manufacturers package and sell the product with an easy name, so you will have to know the scientific name and request it as "Sodium Thiosulfate."
Typically, adding 1LB of Sodium Thiosulfate per 100,000 gallons of water will lower the chlorine level by 1.0 ppm. When using Sodium Thiosulfate, caution must be exercised. If too much of the product is added to the pool, the chlorine level will drop excessively. It will then be extremely difficult and extremely frustrating to revive a sufficient chlorine level. Also, remember that Sodium Thiosulfate will only lower chlorine levels. It will not lower Cyanuric Acid levels; Cyanuric Acid can only be lowered by draining the pool, either partially or completely, and starting over.
NOTE: If the chlorine level stays too high for an extended period of time, the first chemical you need to have tested is the Cyanuric Acid level. If Cyanuric Acid is above 100 ppm, the chlorine level will consistently be high. To rectify this situation, you would need to drain your pool either partially or completely, and start over with fresh tap water. If you drain your pool completely, you will need to again add Cyanuric Acid (which is typically packaged and sold as either "Conditioner" or "Stabilizer"); there is only a trace of Cyanuric Acid present in tap water, and you need a reading of at least 30 ppm in order to protect chlorine from the sun.
SHOCK AND CHLORINE: Chlorine is very effective at killing bacteria, living organisms, and other contaminates (which again are dirt, debris and algae spores), but when ammonia is present, chlorine will combine with it, but cannot successfully kill it, and the two combine to form "Chloramines." Chloramines will remain in the water, and will register as chlorine, but are ineffective at sanitizing, disinfecting, or oxidizing.
Ammonia is commonly present in pool water from either environmental factors, a build up of fertilizers that are carried by wind and dropped into pools, from swimmer wastes (perspiration, urine, saliva and body oils), or even from some suntan lotions. No matter how ammonia enters a pool, it will exist.
So then, since chloramines are ineffective and undesired forms of chlorine, how are they removed from the water? Easy-by shocking the pool on a consistent and regular basis. Even if you maintain a 2.0 - 3.0 ppm (ideal) chlorine reading at all times by merely adding chlorine tablets or granular chlorine to the pool, the presence of ammonia will lead to Chloramines, preventing the chlorine from being fully effective. The only way to combat Chloramines, and kill ammonia, is by shocking the pool.