Chloramines – Simplified
Dougall Stewart BSc.
UK Discus Association Founder
Fred Goodall

Monochloramine (NH2Cl) is an inorganic chloramine that is commonly added to tap water by local water authorities.  It is a disinfectant agent that is added at approximate concentrations of 1.5-2.5mg/l in the UK.  However depending on the amount of ammonia to chlorine present and the pH of the water that the products are added to, other inorganic chloramines may form e,g, dichloramine (NHCl2) and or trichloramine NCl3.

Simplified equations for the reaction between ammonia and chlorine:







hydrochloric acid








If the ratio of chlorine to ammonia is 3-5:1 then monochloramine and hydrochloric acid are formed.  However, as the pH of the solution drops below neutral and more chlorine is added, there is a greater tendency for dichloramine and trichloramine to form.  Dichloramine forms when the chlorine to ammonia ratio is between 5-7:1 and the pH is below neutral to approximately 4 and Trichloramine forms when the ratio if greater than 8:1 and the pH is below 4.

How are chloramines added to our water supply and why?

Normally in the first instance chlorine would be added at source.  Chlorine is the preferred additive of many water boards.  It has a much greater ability to nullify bacteria and viruses than chloramine does; however it does have a relatively short lifespan as a disinfective agent and there are growing  concerns that it forms trihalomethanes (THMs) which are carcinogenic substances that have been linked to heart, lung, liver, kidney, central nervous system damage and miscarriages.  By adding chlorine at source, it can rapidly carry out its disinfective role.  The next step is to add the ammonia; this tends to be added further down stream.  By adding the ammonia, monochloramine is formed and the risk of THMs forming is reduced; in addition the temporal disinfective period is greatly increased giving longer protection from deleterious organisms – albeit with less disinfective penetrative power.  It has been noted that the half life for chloramines varies from 1 to 23 days depending on conditions.

Chloramine and its effect on fish

Chlorine and chloramine both cause death in fish by anoxia i.e. the fish are literally starved of oxygen; however, the mechanisms involved differ.  Chlorine is a major irritant and causes rapid degradation of the gills by oxidation i.e. gill tissue is destroyed; whereas, chloramine crosses the gills and directly enters the blood stream of the fish where methaemoglobin is preferentially formed over oxyhaemoglobin  i.e. the chloramine chemically binds with the iron in blood haemoglobin which has a deleterious affect on the ability of haemoglobin to bind with oxygen – thus starving the fish of its oxygen supply.  The amount of chloramine that passes past gills and into the blood is critical in the ultimate effect of the formation of methaemoglobin and the survival chances of the fish.

Research in Canada has demonstrated that the ‘Estimated No-Effects Value’ (ENEV) for chloramine is 0.0056 mg/L for freshwater organisms and 0.0028 mg/L for marine and estuarine organisms.  One exception to these doses is that clam larvae and copepods are extremely chloramine sensitive and have demonstrated 50% mortality (LD50) at 0.005 mg/L chloramine concentrations when exposed to chloramine for between 48-96 hours.

Testing for Chlorine, Chloramine, total Chlorine

Palintest Ltd. make a wide range of professional electronic water testing equipment.  A good quality water testing meter that test for over 37 different types of tests costs between £500 and £700.  Tanita make an economical total chlorine tester (chlorine and chloramine) which retails at approximately £30.00 sterling and is quick and simple to use.  Yamitzu make a multi chemical test kit for approximately £20.00 and there chloramine test measure chloramine from 0-1mg/l.  There are many more types of chlorine/chloramine testers on the market including simple to use dip and test strips but please remember that most equipment available to the hobbyist will have a resolution of 0.01 mg/l or worse; therefore you really must take care when interpreting the results and you should certainly take steps to neutralise the chlorine/chloramine content if you get any reading at all.

Removal of chloramine

There are numerous methods of removing / neutralising chloramine from our water. I have considered the 3 most popular forms.

1.   Run your water through a duo or tri pod set-up.

Duo Pod:

The first pod would contain activated carbon.  The carbon catalytically breaks down the chloramine to ammonia, nitrogen gas and chloride.  The carbon deals with the chloride and a degree of the ammonia.  The second pod would contain zeolite in order to deal with the balance of the ammonia.  The contact rate should be between 5-10 minutes.  There is a lot of hype and nonsense as to the type of carbon used; however, as long as the carbon is resistant to channelling and is changed according to the manufacturers’ directions – it WILL do the job that is intended.

Tripod (adapted CBR/Metal Ex System):

This setup is as above but with the addition of a 1 micron pre-filter to maximise the effectiveness of the carbon.  If you already own a CBR or Metal Ex system – you could simply add an additional zeolite pod onto the existing pre and carbon filters – cost effective and it works.

2.   Reverse Osmosis

Again there is a lot of hype and nonsense on the ability of reverse osmosis units to remove chloramines.  It should be remembered that the flow rate through an RO unit is suited to chloride removal and partial ammonia removal at the carbon pod at up to 3 mg / L.  The ro-membrane will then deal with the majority of the rest of the ammonia released from the catalytic reaction.  If you are worried about any residual traces of ammonia this can be adequately dealt with by the addition of a deioniser pod and or a post carbon pod (in other words you are building a 5 stage reverse osmosis unit that adequately removes chloramines) and hey presto – chlorine and ammonia free water.  There are 2 provisos to this process though.  The carbon and resin must be changed at regular intervals as recommend by the manufacturers’ instructions – this is especially important if the entering water has a very high (9) initial pH (this causes the membrane pores to swell and the rejection rate to decrease).  Ideal non chemical removal of Chloramines could be accomplished with 2 carbon premembrane pods and the newer TFC or "extruded" RO membranes.  The "extra" Carbon pod assures the necessary contact time to remove Chloramines prior to contact with the RO membrane which will remove the chlorides and ammonias catalized from the Chloramines by the carbon.

3.   Proprietary Chemical Reducing Agents

Sodium Thiosulphate - inorganic sodium thiosulphate is incredibly inexpensive and fast working.  It is added to the chloramine containing water.  The thiosulphate destroys the chloramine molecule and also effectively neutralise chlorine, unfortunately the resulting ammonia still needs to be dealt with if your tank is running at a very high pH.  In a discus tank of pH of 6 this ammonia would be converted to the much less toxic ammonium and would over time be consumed by the filter bacteria etc.

S2O3 + NH2Cl + H2O ® SO4 + H+ + HCl + NH3

Hydroxymethanesulphonate – this is an alternative product and one that is more favourable than the sodium thiosulphate.  It readily breaks down the chloramine, neutralises chlorine and binds up the ammonia.  It is found in Amquel, and I suspect Ammo-Lock 2 – a product I have personally used with success.


I would add one point of caution to the above products in that whilst they do the job they claim to do there is still some concern on the resulting by products and their effect on developing eggs, fry, larvae etc; therefore if you are a breeder, you may want to consider alternative methods to the use of chemicals.


Further Reading

Canadian Department of the Environment, Department of health, (2000), ‘Canada Gazette’, CEPA Environmental Registry, Vol 134: No. 28

Gergely, A., Nichols; R., (1985), ‘Composition and method for removing chloramine from water containing same’, US Patent 4,554,261

Grothe, D., Eaton, J, (1975), ‘Chlorine-induced mortality in fish’, Transactions of the American Fisheries Society, 104, pp 800-802

Hankin, S, (2001), ‘Chemicals in drinking water: chloramines’, Scottish Centre for Infection and Environmental health, Glasgow.

Home-Farley, R., (2003), ‘Chlorine and the Reef Aquarium’, Reefkeeping Magazine™

Lenntech (2003), ‘Lenntech Disinfectants Chloramines’,

GE infrastructure  Water & Process Technologies  (1997 )  "Chloramines",

Sans Francisco Public Utilities Commision, (2003), ‘In Depth Chloramination Q&A: Impact on Animals and Environment’, Sans Francisco Public Utilities Commission

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