Dealing with chlorine and chloramine

Chlorine and chloramine. All water supplied by municipal water companies in the US is chlorinated or chloraminated. The first step in bringing this sterile water to life is de-chlorinating it.
The chlorine ion dissolved in water is hypochlorite (Cl2O2), the same ion as found in regular household chlorine bleach like Clorox. There's no negative anion without a positive cation, so hypochlorite comes to you as sodium hypochlorite (NaCl2O2). When a solution of chlorine bleach in water has entirely dried, there's no residue but a bit of common salt (NaCl) and alkali washing soda (sodium carbonate Na2CO3), which you might want to rinse away. Water utilities have replaced hazardous chlorine gas (Cl2), which used to be compressed to a liquid and shipped around the country in railroad tank cars, either with chlorine produced on-site by the electrolysis of salt brine, which separates the Na from the Cl, or with dry or wet calcium or sodium hypochlorite.
If the chemistry of chlorine is mysterious and you want some basic introduction to the science of chlorine, its manufacture and its everyday uses, the website of the Chlorine Chemistry Division, a chemical industry  trade association, offers some material, including some more specialized studies and discussion (from the chlorine manufacturers' point-of-view of course) of some public policy issues. AquaScience Research Group, manufacturers of a dechlorinator for aquaculture, offer another good discussion of the water chemistry of chlorine and chloramines.
Dechlorinating it, then,  is often the aquarist's first concern with tapwater. Most commercial dechlorinators are based on plain sodium thiosulfate (Na2S2O3), a crystalline salt that generally comes pre-mixed with distilled water, usually in a 1% solution. At this strength, 10 drops (that's 0.5 cubic mm) will neutralize common municipal levels of chlorine in 10 gallons, turning the chlorine to harmless chloride ions and adding some molecules of sodium and sulfur to the water. Unreacted sodium thiosulfate that may be left over is pretty inert and harmless. Two commercial brands that are plain sodium thiosulfate are Wardley's ChlorOut and Mardel's MarChlor.
Other forms of sodium thiosulfate. If you needed large quantities of chlorine neutralizer, you could buy sodium thiosulfate less expensively in crystal form, directly from a manufacturer, such as Fishy Farmacy. At KoiVet Doc Johnson offers a brief, level-headed introduction to dechlorination. If you're thinking of getting sodium thiosulfate in undiluted form, better get together with a few friends, because just 1 gram of crystals in a liter of distilled water makes a1% stock solution. At rates of one drop of this stock solution to a gallon or two of chlorinated tapwater, that could be a lifetime's supply.
There are also sources outside the aquarium hobby, for sodium thiosulfate has other uses. It  keeps down extraneous bacteria in the making of red wines; in contact with the acids in grape juice it forms sulfur dioxide. So, if you want to save money on chlorine "remover" you might want to buy it as a wine-making supply. Or as an antidote kit for cyanide poisoning, by the way! Back when amateur photographers were developing their own b/w photos, sodium thiosulfate was their "developer," and so it was cheaply available as "hypo" in any photo hobby shop. "Hypo" was short for "hyposulfite of soda" an obsolete term for Na2S2O3. Technology marches on, however.
Outgassing chlorine. Before municipal water utilities in the U.S. were switching over to chloramines, I'd have suggested that you just let chlorine outgas naturally. I'm currently still able to do this with New York City's vaunted tapwater. The natural way to dechlorinate is to let the tapwater stand for twenty-four hours in jugs that offer a large water-to-air surface. I like the 6-gallon unbreakable plastic jugs Poland Spring water was delivered in til recently; they're built to be easy to grip and they're rectangular to stash close. But you might need a virgin 55 gal garbage can, which you have indelibly marked in big letters "Aquarium Use Only So Dont Even Think About It."
Either way, the chlorine will dissipate. Frankly, if you have an aerator attached to your faucet, it may provide all the outgassing that's needed for a partial water change; after all, many people are using Python-type water-change hoses without trouble.
And if the faucet aerator is charged with an activated carbon filter to improve the drinking water, so much the better! Fresh carbon will adsorb chlorine, if you can slow the flow from the faucet. If you are impatient you could run an airline in the jug or can, but there isn't any reason to use de-chlorinator to neutralize chlorine (but not chloramine), except in an emergency. Don't let anyone undermine your security about this fact. If you have any lingering doubts, borrow someone's chlorine test kit (don't buy one yourself) and test your water. Test the water that has passed through your faucet aerator. And test the water first from the tap and again after sitting still for twenty-four hours.
If you insist on owning your very own chlorine test, by all means get it at the Swimming Pool Supplies section of your Home Depot. It's the very same test, using the very same chemicals, as ones that are specially packaged and specially priced for the "captive" aquarium market.
Chloramines. Chloramines offer a more aggressive treatment for maintaining some residual chlorine in tapwater. Unlike chlorine, chloramine won't dissipate. In fact that's why water companies use it: chloramine remains more stable in the water mains than chlorine. In areas where organic molecules in drinking water are high, chlorine tends to bind with them, even such harmless ones as humic or fulvic acids, to form trihalomethanes, which are implicated in cancer. Chlorine will bind with phenols too, if they are present, to give a foul chemical taste. Trihalomethanes could simply be adsorbed by activated charcoal at the water plant, according to the McGraw-Hill Encyclopedia of Science and Technology article "Water Treatment." But in order to be effective, activated carbon needs a slow flow that offers sustained contact with the water and frequent reactivation in a kiln. On the giant scale that's required, carbon filtration isn't practical. So instead, US water boards are increasingly adding chloramines before water leaves the treatment plant, acting under pressure from the E.P.A. who lowered permissible standards for trihalomethanes in November 1998.
"Chloramine is formed when ammonia is added to water that contains free chlorine. Depending upon the pH and the amount of ammonia, ammonia reacts to form one of three chloramine compounds. Of the three, monochloramine is the preferred compound." So says the standard FAQ sheet distributed by many local US water authorities: for example Jefferson County (Missouri) Water Authority, 2010. Because the chloramines are much more stable than chlorine, they maintain better residual disinfectant levels in the water mains. The stability of chloramine creates problems for fishkeepers, since these chemicals will not simply outgas in a holding can, the way chlorine does. Even exposed to sun and plentiful oxygen, Chloramine-T could still last for as long as a week. My understanding of the FAQ sheet's quote is that some chloramines could as readily form in aquarium water if you were to add chlorinated water straight from the tap to a tank that already carried some free ammonia. Which chloramine formed would depend on the pH of the water, a factor which controls the interconversion of ammonia (NH3) with ammonium (NH4). Chloramine formation could only become an issue if you were using a "direct-fill" hose, and had highly-chlorinated tapwater and free ammonia in the tank.
In chloramine, two chloride ions are bound to each ammonia molecule, and that's why you're usually advised to double the quantity of sodium thiosulfate you'd use for chlorine alone. In acidic water, the ammonia released would largely be ionized to its non-toxic form, ammonium. In a planted aquarium NH3/NH4 would be rapidly scavenged by the plants.  
Chloramine toxicity. Chlorine is an oxidizer, which burns a fishes' gills. Chloramines, on the other hand, pass across the gills of a fish and into its blood, where the molecule attaches to the hemoglobin, acting like nitrite to induce methemoglobinemia. The toxicity of chloramines is affected by pH, I'm reading at FishDoc, with Chloramine-T more toxic at lower pH. Fish stricken by chloramine poisoning are sluggish and respire heavily.
"Deadly"? chloramine. But chloramines have been inflated into a bugaboo by some packagers/distributors of various water "conditioners." Aquarium Pharmaceuticals, for instance, characterizes chloramine as "deadly" in corporate literature. Nevertheless, the not-invariably-"deadly" Chloramine-T is currently being studied by the U.S. government as potentially important to fish hatcheries in controlling bacterial gill disease. Studies at UC Davis have inspired widespread use of Chloramine-T to kill pathogenic bacteria and parasites in koi ponds. A professional assessment I trust is this from John P. Grazek (in Aquariology: Fish Diseases and Water Chemistry, Tetra Press 1992): "The addition of sodium thiosulfate will neutralize both chlorine and chloramine. However, ammonia is released when the sodium thiosulfate combines with the chloramines, and this could be a problem to fish where there is little or no biological filtration." 
Testing for chloramines. If you're testing for chloramines, make sure the test kit you've borrowed is testing for "total chlorine" or "combined chlorine," not for "free chlorine." A test for "free chlorine" would misleadingly read zero in chloraminated water.
On the other hand, when your tapwater tests positive for ammonia, this is a sign that your water is being treated with chloramines. The Washington DC water utility offers a document "Tap water and fish" generated by the U.S. Army Corps of Engineers, which injects a note of sobriety into this sometimes panic-inducing situation. Being a public agency, the Washington Aqueduct couldn't recommend any commercial brand, but in general they recommended four general methods for neutralizing chloramines: 1. activated carbon in filtration, 2. sodium thiosulfate, 3. commercially-available de-chloramination products ("some simply remove the chlorine, while others 'lock up' or detoxify remaining ammonia"), or 4. a chemical agent plus a biological agent ("bio-filter") to remove the ammonia. (You should already have known all this, eh?)
If you're depending on 1. filtration with granular activated carbon to break the chloramine bond, make sure the carbon is fresh and the filtration is slow. Since some ammonia is likely to be freed, one way or the other, you have an additional incentive to de-chloraminate before you add water to the aquarium. If you're de-chloraminating as in 3. with commercial products, it's useful to know that Ammo-Lock2 (Aquarium Pharmaceuticals) and AmQuel (Kordon) each react with the ammonia to form non-toxic, inert, moderately stable substances. With these products, the ammonia is bound, but not actually removed. It does remain available to the nitrifying bacteria, I understand; that's an important consideration. Each company presents a clear un-hyped analysis of its product, Kordon at and Aquarium Pharmaceuticals at


I am skeptical about the necessity of doing anything about chloramine for typical water changes, at least here in Brooklyn. My friend, in a different Brooklyn neighborhood, suggested that he changes up to half his water at a time, never uses any water treatment, and never had problems. I've been doing the same now for years, also with no problems, (no sluggishness, no heavy breathing, no more likely fish deaths in the days after a change). Are there other symptoms I should look for? Are we just lucky around here?

wetman's picture

We're all lucky! there's no chloramine in New York City water. We have no experience with it. Our chlorine outgasses, and partial water changes, even as much as 50%, dilute its effects.


It is chlorine gas dissolved in a solution of lye (high pH water) that produces hypochlorite ion, not "chlorine ion" whatever that is (did you mean chloride ion?). The chemical formula for hypochlorite ion is not Cl2O2, but ClO- (or OCl-), and the formula for sodium hypochlorite is not NaCl2O2, but NaClO (or NaOCl). The electrolysis of salt brine does not separate Na from the Cl. When salt (NaCl) dissolves in water, the sodium (Na) is separated from the chlorine (Cl) to form separate sodium and chloride ions (Na+ and Cl-). The electrolysis does not directly chemically involve the sodium except for charge balance as an electrolyte. The electrolysis converts chloride ion into chlorine gas which bubbles out and is captured to be added to an alkaline solution of lye as follows:

2H2O + 2e- ---> H2(g) + 2OH-
Water + electrons ---> Hydrogen Gas + Hydroxyl Ion
2Cl- ---> Cl2(g) + 2e-
Chloride Ion ---> Chlorine Gas + electrons
2H2O + 2Cl- ---> H2(g) + Cl2(g) + 2OH-
Water + Chloride Ion ---> Hydrogen Gas + Chlorine Gas + Hydroxyl Ions

The brine solution becomes more alkaline and sodium hydroxide (aka lye or caustic soda) is extracted from it. The chlorine gas is captured and then added to a purified water solution of sodium hydroxide as follows:

Cl2(g) + 2NaOH ---> NaOCl + NaCl + H2O
Chlorine Gas + Sodium Hydroxide ---> Sodium Hypochlorite + Sodium Chloride + Water

The above is known as the chlor-alkali process where you can see why bleach and chlorinating liquid contain sodium chloride salt in addition to sodium hypochlorite.

As for chloramine, it is not true that for monochloramine, which is what is in some municipal water supplies, has two chlorine. It has only one as the formula is NH2Cl and comes from the combination of chlorine with ammonia:

HOCl + NH3 ---> NH2Cl
Hypochlorous Acid + Ammonia ---> Monochloramine

or equivalently in higher pH solutions:

OCl- + NH4(+) ---> NH2Cl
Hypochlorite Ion + Ammonium Ion ---> Monochloramine

There are inorganic chloramines with more chlorine such as dichloramine, NHCl2, and nitrogen trichloride (trichloramine), NCl3, but these are intentionally minimized in municipal water supplies.

As you pointed out, the problem with using sodium thiosulfate (or other reducing agent) or a carbon filter with monochloramine (i.e. chloraminated water) is that it produces ammonia which must be dealt with separately.

There is another way you can remove chloramine in the water and that is to oxidize it by adding more chlorine to it. This will produce mostly nitrogen gas and some nitrate, but also a small amount of nitrogen trichloride that is volatile. After the chloramine is oxidized, you can then dechlorinate the leftover chlorine in the water. The reaction of 2 ppm Free Chlorine (FC) with 1 ppm Combined Chlorine (CC) as monochloramine (0.2 ppm-N, a typical amount in chloraminated water) at a pH of 8.0 is over 99% complete within 1 hour at 77ºF (if you use only 1 ppm FC per CC, then it's only 90% complete in an hour). At that point you can dechlorinate the water which has around 1.2 ppm FC leftover (i.e. about 0.8 ppm FC got used) while the ppm-N of remaining ammonia will be less than 0.01 ppm-N. So the chlorine dosing rule for this procedure to remove the ammonia from monochloramine is twice the CC level measured in a chlorine test kit or ten times the ppm-N level measured in an ammonia test kit (which also measures monochloramine as ppm-N, especially for the salicylate ammonia test). The sodium thiosulfate dosing rule for getting rid of the leftover chlorine is the same as usual (or you can use a carbon filter at that point, if you prefer).

wetman's picture

...for tightening my loose grip on the chemistry.


Regarding the appropriate dilution of sodium thiosulfate, the above statement "...because just 1 gram of crystals in a liter of distilled water makes a1% stock solution" is incorrect. By definition, 1g solute in 100 mL water will provide a 1% solution.