Chloramine is a disinfectant put into many municipal water supplies. In recent years it has often replaced chlorine for two main reasons. The first is that it is much longer lasting, so it continues to provide a disinfectant action in supply pipes, where chlorine typically loses its capacity to disinfect. The second is that it does not react with organics nearly as readily as does chlorine. The reaction products of chlorine and organics (chlorinated organics) are very toxic to people, and water supply operators elect to use chloramine to reduce this toxicity.

Unfortunately for aquarists, dealing with chloramine in tap water is not as easy as dealing with chlorine. The chlorine in tap water can be eliminated just by letting the water sit for a few days prior to use. This is not the case for chloramine, and aquarists MUST take active steps to eliminate it.

This article describes what chloramine is, what it does that is a problem in aquaria, how to test for it, and how to rid water of chloramine. It also reports on a survey study of aquarists using reverse osmosis/deionization systems (RO/DI) for purification of their water. There has recently been considerable concern and debate over whether such systems will adequately remove chloramine in all normal circumstances, even among manufacturers and distributors of such systems. The results of the survey described here will help aquarists understand how effective such systems are at removing chloramine.

What is Chlorine?

Before beginning to discuss what chloramine is, understanding chlorine is useful. Chlorine, as Cl2, is a greenish yellow gas at room temperature. It is sometimes used as a disinfectant in water supplies, and it is also used to make chloramine, as described below. When dissolved in water, it forms dissolved Cl2, and it also reacts with water to form HOCl (hypochlorous acid; pKa = 7.5), and HCl (hydrochloric acid). The HOCl and HCl will also dissociate into H+, Cl-, and OCl- (hypochlorite), with the extent of dissociation depending on pH.

Cl2 + H2O HOCl + H+ + Cl-

Since chlorine and hypochlorous acid/hypochlorite are in equilibrium in water, it doesn't really matter which ones are added to attain a disinfectant medium. So, for example, one can add chlorine gas, hypochlorous acid, or hypochlorite, and attain similar results. In fact, according to the U.S. Environmental Protection Agency (EPA),

"The term free residual chlorine most accurately refers to elemental chlorine, hypochlorous acid (HOCl) and hypochlorite ion (OCl-)."

In light of that, many water supplies (including the Massachusetts Water Resources (MWRA) that serves my area)1 choose to use sodium hypochlorite (bleach, NaOCl) to provide this same OCl- as the primary disinfectant.

What is Chloramine?

Chloramine is formed through the reaction of dissolved chlorine gas (forming hypochlorous acid) and ammonia in tap water. Chloramine is a term that actually describes several related compounds: monochloramine NH2Cl (Figure 1), dichloramine, NHCl2 and trichloramine, NCl3:

NH3 (ammonia) + HOCl NH2Cl (monochloramine) + H2O

NH2Cl + HOCl NHCl2 (dichloramine) + H2O

NHCl2 + HOCl NCl3 (trichloramine) + H2O

Figure 1. A 3-dimensional model of monochloramine showing the relative size
of the chlorine, nitrogen, and hydrogen atoms.

The predominate form in most water supplies (where the pH is 7 or above) is monochloramine, and that form will be chosen for most discussions in the remainder of this article. and that form will be assumed to exist in the remainder of this article. Nevertheless, water supplies may contain mixtures of these compounds, and the exact proportions of the various species present depend on the pH and the relative concentrations of chlorine and ammonia when reacted.

How much chloramine is used by water supplies varies quite a bit. In the case of the water that I use, for example, the MWRA uses chlorine for primary disinfection (via sodium hypochlorite), and then later uses chloramine to provide lasting disinfection as it is sent into pipes.1 In the latter case, the amount of chloramine added may not be as high as if it were used for the primary disinfection. Most aquarists in the greater Boston area (including myself) found chloramine levels of less than 0.5 ppm-Cl in their tap water when tested at a recent Boston Reefers Society function. Elswhere, however, chloramine levels can be significantly higher, ranging up to several ppm-Cl. The maximum allowed by the EPA is 4 ppm-Cl , and some water supplies target 2-4 ppm-Cl . The amount seen at the tap will also depend on distance from the treatment plant, and how long the water has been sitting in pipes.

A note on concentration units. In this article (and in links to the EPA and other sites), all concentrations are given as ppm-Cl. That means ppm of chlorine mass, regardless of what form the chlorine is in. It is analogous to units of NO3-N (nitrate nitrogen) that are often used for nitrogen species. That complication is necessary since the various chloramines may be present as mixtures, and it also facilitates comparison to chlorine and other oxidants. So 1 mg/l of monochloramine would be reported as 0.69 ppm-Cl of monochloramine because the chlorine comprises 69% of the mass of monochloramine. The unit does not imply that there is any free chlorine present.

Toxicity of Chloramine to Marine Organisms

The great majority of reported toxicity tests involving chloramine use freshwater organisms. Nevertheless, there is adequate testing reported for a variety of marine organisms to know that it is very toxic to such organisms.2-7 A complication in marine systems is that chloramine and chlorine react with substances in seawater, forming other reactive chlorine species. In the case of chlorine, these are often simply referred to as chlorine-produced oxidants (CPOs). For example, monochloramine is known to react with bromide in seawater over a period of hours to form bromochloramine (Br-NH-Cl).8 Consequently, identifying the exact species causing toxicity is often difficult.

What is the mechanism of toxicity? The mechanism has not been established for many invertebrates, but in fish the mechanism is well known. Chloramine passes through the gills of fish and enters the blood stream. There, it reacts with hemoglobin, forming methemoglobin. In fathead minnows (Pimephales primelas) exposed to 1 ppm-Cl of monochloramine, for example, about 30% of the hemoglobin is converted into methemoglobin. The fish then suffer from anoxia (low oxygen in their tissues) because they have lost some of their hemoglobin, which is responsible for carrying oxygen in the blood.9

While knowing the exact species causing the toxicity is important to physiologists studying the phenomenon, it is not so important to aquarists. The important thing that aquarists need to know is how low the concentration needs to be before toxicity is not displayed by any organisms that are present in the aquarium. Most toxicity tests are designed with unmistakable upper endpoints, often death. Table 1, for example, shows the concentration that kills half of the exposed individuals in a few days, called the LD50.

Table 1. Toxicity of chloramine and CPOs to marine species.

Species Concentration to kill half of the individuals in 48-96 h Concentration to kill half of the individuals in 168 h
Amphiporeia virginiana (amphipod)3 0.57 ppm-Cl 0.043 ppm-Cl
Eohaustorius washingtonianus (amphipod)3 0.63 ppm-Cl 0.13 ppm-Cl
Syngnathus fuscus (pipe fish)4 0.27 ppm-Cl ----
Crassostrea virginica larvae (oyster)4 <0.005 ppm-Cl ----
Mercenaria mercenaria larvae(clam)4 <0.005 ppm-Cl ----
Acartia tonsa (copepod)4 <0.005 ppm-Cl ----
Natural mixed phytoplankton4 0.1 ppm-Cl ----
Homarus americanus  larvae (lobster)5 0.3 ppm-Cl ----
Homarus americanus  larvae (lobster)6 0.6 ppm-Cl ----
Homarus americanus  larvae (lobster)6 0.05 ppm-Cl caused respiratory distress ----
juvenile killifish5 >0.8 ppm-Cl ----

In its assessment of chloramine toxicity to marine invertebrates, Environment Canada (the Canadian equivalent of the United States Environmental Protection Agency, EPA) determined the Estimated No-Effects Value (ENEV) based on this type of data to be 0.002 ppm-Cl for marine and estuarine environments.

How much chloramine should one allow into an aquarium? That, of course, depends on what is in the aquarium. In the absence of knowing the toxicity of chloramine to every inhabitant of the aquarium (or of even knowing the identity of every inhabitant), it seems prudent to have chloramine levels far below those where the most sensitive organisms are killed, and that chloramine concentration is somewhere well below 0.005 ppm-Cl. The value suggested by Environment Canada seems like a reasonable maximum.

There is, however, substantial uncertainty in deciding exactly which levels are acceptable and which are not, since there is so little data available. Perhaps the acceptable levels for daily exposures during the entire lifetime of an organism needs to be even lower than this value. After all, some organisms live quite a long time, and presumably we are interested in preventing all toxicity, not just death. It is apparent from the data in Table 1 that the longer the exposure, the lower the toxic levels become. In the end, we are limited by the available data and also by the ability of aquarists to measure chloramine itself.

This target of 0.005 ppm-Cl or less does not necessarily imply that all water used for aquaria must be that low. For example, an aquarium that tops off 2% of the tank volume daily (to replace evaporated water) will not have a chloramine concentration equal to the top off water. It will, however, have fresh chloramine added every day. Even if the chloramine added each day is broken down in the aquarium before the next addition (something that is likely, but not demonstrated for aquaria), then if the top off water contained 4 ppm chloramine, the aquarium would be boosted to 0.08 ppm every day. That level appears to be well above the danger zone for many invertebrates. Consequently, aquarists need to be aware of the chloramine levels in water that they use to replace evaporated water. Similar, and even more stringent, concerns would apply to water used for water changes or in setting up a new aquarium.

Measuring Chloramine

There are many kits suitable for measuring chloramine, with varying limits of detection. Many are not suitable for testing the low levels necessary for reef aquaria. The kit that I prefer for measuring low levels of chloramine is the Hach CN-70 (part # 1454200). It is capable of measuring total chlorine and free chlorine. Chloramine is found by the difference between these two values. It has a low range scale that runs from 0 to 0.7 ppm, and a high range that runs from 0 to 3.5 ppm. The low range can detect 0.01 ppm chloramine. It costs about $64 (with shipping) and is good for many tests. The colorimetric kit is very easy to use: reagent is mixed with the water to be tested and compared to a color wheel.

Removing Chloramine From Water: Chemical Reducing Agents

There are two primary ways to remove chloramine from tap water. The first is through the use of inorganic reducing agents such as thiosulfate. Thiosulfate (S2O3- -, which actually looks like -OSO2S-) is an inorganic chemical that is typically dissolved in water, usually as the sodium salt. When added to water containing chloramine, a reaction takes place, destroying the chloramine. The electrochemistry of sulfur compounds can be complicated, and different researchers report different products of this reaction (extrapolated from reactions with chlorine itself, not chloramine). The products have been suggested to include sulfate (SO4- - and HSO4-),10,14 elemental sulfur (S),10 and tetrathionate (S4O6- -),11-13 and may depend to some extent on the conditions, including the pH and the relative amounts of compounds present. John F. Kuhns (inventor of Amquel below) has indicated that he believes that the reaction resulting in sulfate is the most frequently observed. The reaction for this process is shown below:

S2O3-- + 4NH2Cl + 5H2O 2SO4-- + 2H+ + 4HCl + 4NH3

Thiosulfate is also equally suited to dechlorinating free chlorine in water, and it has gained wide use in marine and freshwater aquaria. Unfortunately, the ammonia that is produced as a result of the reaction is still toxic. Consequently, thiosulfate alone is not always adequate for eliminating toxicity from chloramine.

Other products, such as hydroxymethanesulfonate (HOCH2SO3-; a known ammonia binder15 patented for aquarium uses by John F. Kuhns16 (sold as Amquel by Kordon and ClorAm-X by Reed Mariculture, among others) can be used to treat chloraminated water because they both break down chloramine and bind up the ammonia.

The reaction of ammonia with hydroxymethanesulfonate is mechanistically complicated, possibly involving decomposition to formaldehyde and reformation to the product (aminomethanesulfonate; shown below).15 The simplified overall reaction is believed to be:

NH3 + HOCH2SO3- H2NCH2SO3- + H2O

Even more complicated is the reaction of hydroxymethanesulfonate with chloramine, or chlorine (as Cl2 or HOCl). In this case, the products that are formed have not been established.

So are these useful products? That is, do they eliminate all toxicity from chloramine and provide none of their own, either by themselves or through their degradation products? I cannot answer that question. Almost certainly, using them is better than not using them if there is chloramine in the water. Is the toxicity eliminated for even the most sensitive larval invertebrates? Again, I don't know. Without knowing what the degradation products are, or without detailed testing on a variety of very sensitive invertebrates, I don't know how one would conclude that they are satisfactory (or not). Maybe such tests exist, and if so, I'd be pleased to hear of them. In the end, my recommendation is to remove chlorine and chloramine in other ways, such as through an RO/DI system as described below.

Removing Chloramine From Water: Activated Carbon

Another method for removing chloramine from water is with activated carbon (as is contained in most RO/DI systems). In a two step process, the carbon catalytically breaks the chloramine down into ammonia, chloride, and nitrogen gas

C + NH2Cl + H2O C-O + NH3 + Cl- + H+

C-O + 2NH2Cl C + N2 + 2Cl- + 2H+ + H2O

where C stands for the activated carbon, and C-O stands for oxidized activated carbon. In this case, as was found for thiosulfate, the product includes ammonia, which is not bound significantly by activated carbon. Consequently, treatment of water with activated carbon will need to be followed up by some method of eliminating the ammonia.

In the case of a reverse osmosis/deionizing system (where carbon is usually part of the prefiltration prior to the RO membrane), the ammonia is partially removed by the reverse osmosis system. The extent of removal by the RO membrane depends on pH. At pH 7.5 or lower, reverse osmosis will remove ammonia from 1.4 ppm-Cl monochloramine to less than 0.1 ppm ammonia. The DI resin then removes any residual ammonia to levels unimportant to an aquarist.

Removing Chloramine With Activated Carbon: Does it Really Work?

There has been much debate over whether commercial RO/DI systems used by aquarists are actually removing chloramine in adequate quantity. The concern is not whether they can theoretically do so, but whether the actual units allow sufficient contact time between the water and the activated carbon for the units to do an adequate job.

I have been using a Spectrapure RO/DI system (CSP25DI) for years, and my water does contain chloramine, so naturally I was interested to know if it was up to the task. In discussing the issue with Charles Mitsis, President of Spectrapure, he said that my water was among the most difficult to successfully remove chloramine from because the pH was high, and he was not sure that the unit was adequate. The reasons for being concerned were that:

1. Monochloramine is the most difficult of the three chloramine species to remove because it is small (allowing it to pass through a reverse osmosis membrane).
2. Monochloramine is the most chemically stable of the chloramine species, so is the hardest to break down (as on activated carbon).
3. Monochloramine predominates over the other forms in tap water at pH above 7 (dichloramine predominates at pH 4-7).
4. The pores of the activated carbon may become plugged with sediment over time, reducing the effectiveness of the carbon at breaking apart chloramine.
5. At high pH, the pores of the RO membrane can swell, resulting in poorer rejection of impurities.

With this as the backdrop, I set about organizing a round of testing by aquarists to see if their commercially-available systems were adequately removing chloramine.

First, I selected a single, high quality test method for participants to use: the Hach CN-70 kit described above. I then asked aquarists to test several things:

1. The free and total chlorine in their tap water after letting it run for a while.
2. The free and total chlorine in their RO reject water.
3. The free and total chlorine in their finished RO/DI water.
4. The pH of the tap water.

In my case, for example, I had the following results:

Tap water:
pH ~9
Total Chlorine: 0.4-0.5 ppm one day, 0.08 ppm on a second day.
Free chlorine: <0.01 ppm (effectively all of the total chlorine was chloramine)

RO Reject water:
Total Chlorine: 0.02 ppm
Free chlorine: <0.01 ppm

Final RO/DI water:
Total Chlorine: <0.01 ppm

Consequently, within the capabilities of the Hach test kit (0.01 ppm), there is no chloramine getting through the system. A small amount does appear to get past the carbon to the RO waste water, but it does not get through the RO membrane and DI resin.

A similar set of data (more or less complete) was collected from about 20 aquarists in different parts of the country. These included systems that were stated to have a capacity of 25-100 gallons per day, the higher volume systems being especially interesting because the contact time with the carbon might be shorter. All but one had similar results to those reported here. The anomalous report produced the following results:

Tap Water:
pH 8.2
Total Chlorine: >3.5 ppm
Free Chlorine: >3.5 ppm

Filtered Tap Water: (single cartridge under sink, cold water side)
Total Chlorine: 0.7 ppm
Free Chlorine: 0.38 ppm

RO water: (11 month old cartridges)
Total Chlorine: 0.16 ppm
Free Chlorine: 0.06 ppm

RO/DI water: (11 month old cartridges)
Total Chlorine: 0.04 ppm
Free Chlorine: 0.02 ppm

RO/DI water: (Fresh cartridges)
Total Chlorine: <0.01 ppm
Free Chlorine: <0.01 ppm

In short, his tap water chloramine (and chlorine) levels were quite high. His old carbon and sediment cartridges were not quite up to the task, but when replaced, were adequate to remove all of the chloramine. Note that the 11 month old cartridges were still producing 0-1 ppm TDS RO/DI water.

Lessons Learned and Suggestions:

1. Most RO/DI systems seem capable of removing chloramine adequately for aquarists.
2. The carbon cartridge may become less useful over time, and it is possible that the chloramine removal effectiveness of a system may be lost before the DI appears to need changing.
3. Cheap sediment cartridges may expose the carbon cartridge to unnecessary fouling, which may permit chloramine to pass through the system. Cartridges should be replaced as soon as the pressure drops significantly, even if RO/DI water is still being produced at a reasonable rate or purity as measured by total dissolved solids.
4. Testing for chlorine and chloramine is easy, so any concern is easily reconciled.
5. One Hach kit provides several dozen test results. Our local Boston Club bought some kits and had a "water testing day." The kits can also become part of the "library" of a local club for aquarists to use once in a while to see if their systems are functioning. That way, the cost to each aquarist is minimal.


Chloramine in tap water should be a significant concern to aquarists. Its peculiar properties make it well suited to disinfection of water supplies, but also make it a potential toxin in aquaria. In order to render the water safe for use, aquarists need to use one of two systems for purification: an inorganic reducing agent combined with an additive that binds ammonia (or a single product that does both), or an RO/DI system. Chloramine is toxic enough that it would seem prudent for aquarists to spend the time and money necessary to ensure that they do not unduly stress their organisms. This activity includes setting up appropriate purification systems, and may also include testing the water to ensure that those systems are functioning properly.

Happy Reefing!

If you have any questions about this article, please visit my author forum on Reef Central.


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8. Kinetics of monochloramine decomposition in the presence of bromide. Trofe, Timothy W.; Inman, Guy W., Jr.; Johnson, J. Donald. Sch. Public Health, Univ. North Carolina, Chapel Hill, NC, USA. Environmental Science and Technology (1980), 14(5), 544-9.

9. Chlorine-induced mortality in fish. Grothe, Donald R.; Eaton, John W. Dep. Ecol. Behav. Biol., Univ. Minnesota, Minneapolis, MN, USA. Transactions of the American Fisheries Society (1975), 104(4), 800-2.

10. Dose of Thiosulfate needed in dechlorination of water. Al'terman, N. A. Med Inst., Stalinsk, gigiena I Sanitariya (1958), 23(No 6), 66-7.

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12. The reactions between bleaching powder and thiosulfate in the purification of potable water. Strunk, H. Veroeff. a. d. Militaersanitaelsw. (1914), 28.

13. A study of analysis errors caused by nitrite and free available chlorine during iodometric titration of total residual chlorine in wastewater. Dietz, Edward A., Jr.; Cortellucci, Remi; Williams, Mary. USA. Water Environment Research (1996), 68(6), 974-980.

14. A potentiometric study of the reaction between halogen solutions and sodium thiosulfate. del Fresno, C.; Valdes, L. Anales soc. espan. fis. quim. (1936), 34 813-17.

15. Mechanism of the reaction of ammonia with the bisulfite derivative of formaldehyde. Henaff, Philippe Le. Compt. Rend. (1963), 256 3090-2.

16. Method and Product for removal of chloramines, chlorine, and ammonia from aquaculture water. Kuhns, John F. US Patent #4,666,610; May 19, 1987.

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Chloramine and the Reef Aquarium by Randy Holmes-Farley -