V. You & Chlorine or Chloramines

You & Chlorine or Chloramines


In the United States, most of us get our water for home and aquarium use from our taps. Our taps are supplied with water by a local community, city, or area utility. Those utilities are highly regulated by state and local governments, which are in turn under regulation by the EPA, the Environmental Protection Agency of our federal government. The EPA sets the rules under which our local utilities operate. They define what agents are used to process and disinfect our water supply, and how much of the agent can or must be used.

Public water supplies in the USA provide potable (safe to drink) water through a variety of processing steps and procedures. The early steps vary with the water source. Those early steps are not of immediate concern to the hobbyist, but the water source itself does have impact on the profile (to be detailed in a separate article) of the water you have in your tanks. After the primary in-plant processing, a secondary disinfectant is added to protect the water in and through the pipeline network which supplies each residence or business served by the utility. This secondary disinfectant is the subject of this note.

The “Background” section below is fairly heavy going, as it deals with regulation and compliance, and some details of the two disinfectants we all know by reputation. This section is for those interested in more detailed information, but is not at all required reading to be able to handle either of the two disinfectants before our fish are exposed to them. Personal choice here, read if you wish – even if only to see how difficult and expensive it can be to improve public safety – or skip down to the specific disinfectant handling after the background information section. Those last two parts, under the disinfectant names, have the required information on handling whichever disinfectant you have in your water. If you do not know which you have, you can call your local utility directly. Their number will be listed under the listings for the city or county government of the area where you live. Unlike many government offices, water utilities seem to welcome consumer inquiries – a refreshing change in itself.


Prior to the last quarter of the 20th century, the standard secondary disinfectant used to protect potable water through the pipes to the user’s taps was dissolved chlorine gas (CL2). Chlorine is a halogen, it is highly reactive, a gas at normal tenperatures, and is highly toxic to most life. But it is a moderately good disinfection agent in water and dissolves well enough in water (see reference 1, below). The concentrations set by the federal government regulations are low enough to be relatively safe for human consumption. This process was reevaluated due to the discovery that chlorine gas in water could produce potential carcinogens of then uncertain risk to people. After many tests and evaluations, the Environmental Protection Agency (EPA) selected chloramine or chloramines (several forms are commonly generated in processing, see reference 2 below) as the new preferred secondary disinfection agent. Chloramine(s) had previously been used in a number of municipal systems early in the 20th century, but that use was discontinued due to difficulty in obtaining the necessary materials. That was long before the federal government took overall control of public utility water processing. The relatively recent changeover could not be immediate or even at all rapid. It requires utilities to reconfigure their water processes and provide for safe storage, handling, and the use of new and hazardous raw materials, and for them to purchase the new equipment needed for the new process, and then to test that new process exhaustively through EPA oversight and final approval before it could go on-line to the public water supply. This required a long change-over era, which is still in process. The emphasis was and still is that large cities should change as quickly as possible to protect the greatest number of people. Any smaller utility requiring extensive equipment replacement and all new processing plants of whatever size should be equipped with the new process, not just the out-of-date one. That is the current situation. Many cities have changed over, but some may still have mixed use depending on the number and age of the individual plants in use to serve the public. Quite a number of relatively new but smaller utilities are also using the newer process. Currently more that 50% of the population has chloraminated water. For greater and more detailed information on chlorine and disinfection, see references 1 & 4 (below).

There were a number of secondary issues as well with chlorine use. If some branch pipelines had periods of low use or had smaller populations than anticipated when the pipeline were installed (i.e., are over-sized), in both those cases the result is extended dwell-time in the network. This could result in the chlorine gas being broken down prior to the water’s use, or escaping through small leaks. This could and does leave the water in such pipes below the required chlorine levels to ensure safety. In case of line breaks, or routine new connections to the pipe, residual chlorine in distant-from-the-plant lines could easily be too low for safety. Chlorine is still in use in many areas, but its shortcomings are not well known. The potential and often real issues may require the utility to boost concentrations in the network, with connections near the processing plant getting higher disinfectant levels than in the past, but still within EPA limits, to ensure that distant connections have adequate protection.

Chloramines, on the other hand, are non-volatile and soluble (stay in solution, neither evaporate nor fall out of solution) much longer than dissolved chlorine gas. They are not implicated in the production of potential carcinogens. It is effective as a disinfectant. It doesn’t even have the strong odor, as dissolved chlorine gas does. But the same features which make it great for water protection also make it more dangerous for keepers of wet pets. We cannot handle chloraminated water in the same ways, or by the same treatments, as we used for dissolved chlorine gas. The use of only such treatments can kill fish. For more on chloramines, see references 2, 3, 5 (below).

Chlorine Gas:

This highly reactive halogen gas is volatile enough that can be easily detected by its odor, especially in the shower or when aerating faucets are used. This is one of chlorine’s short-comings as a disinfectant: It off-gases (volatilizes) from exposed water. Hobbyists have made good use of this effect for many years. Chlorinated tap water, especially drawn through an aerating faucet, will off-gas and effectively lose all its chlorine to the atmosphere within days. Some hobbyists may not fully understand the off-gassing process and may not use the most effective setup for off-gassing. The best process is an open-top container with a powerhead or pump to circulate the water, or even just an airstone. Preheating the water to tank temperature will improve off-gassing slightly as well. This obviously calls for a relatively large container, but it also means that fewer containers are needed, as the circulation greatly enlarges the effective surface area for off-gassing. Exposed surface area is critical. The best situation without circulation in theory could be shallow trays with large surface exposed to room air, but that is impractical in application – it would be very messy and require large amounts of space. Buckets are acceptable, but not overfilled, please. If bottles must be used, do not fill past the shoulder (where the bottle starts narrowing) – this will allow the largest possible surface exposure. I used spare tanks or food-safe plastic tubs (trash can scale), both with pumps and heaters, open-topped. I have never detected residual chlorine after 24 hours operation in these, but allowed 48 hours for safety and to remove the requirement for routine testing. Static containers may or may not be safe to use after just 24 hours. Most, with good surface area exposed, will be after 48 hours, but this is best confirmed by test. If after you have found the required time for off-gassing, then you can add a bit more to ensure removal and no longer routinely test so long as the utility does not change the concentration. We no longer have hobby liquid tests for chlorine or chloramine, but must rely on swimming pool tests.

If you do not have the space and time to off-gas chlorinated water, there are many products available which will “neutralize” the dissolved chlorine. The active ingredient historically was sodium thiosulfate, and it is still highly effective for this use. This material captures any free dissolved chlorine gas and coverts the elemental chlorine (Cl2 dissolved gas) to the chloride ion (Cl-) which is harmless at those concentrations. The reaction is rapid. Just add the recommended amount, stir very briefly and add to the tank.

With dissolved chlorine gas disinfectant, there is only one job to be done, and it can be accomplished in two ways: Remove the chlorine gas (off-gassing), or inactivate it (chemical conversion to the chloride ion by thiosulfate). These are simple and straightforward.


The hobby situation with chloramines is more complex and demanding. We cannot efficiently off-gas chloramines, so the simplest solution with chlorine does not apply at all. We equally cannot use just thiosulfate – it does not do enough. There are 3 separate and distinct jobs, all of which must be done to ensure the safety of chloraminated water for use in our tanks:
1. Break the chloramine-ammonia bond. Thiosulfate alone can do this at about the same dosage used for chlorine-only disinfectant.
2. Convert the freed dissolved gas chlorine (Cl2) to chloride ion (Cl-). Thiosulfate again can do this as well; at about the same dosage as before, so double the chlorine-only dose can do both of these two jobs well.
3. Lock the freed ammonia dissolved gas (NH3) into the ammonium ion (NH4+) form (which is usable by the nitrification bacteria). The former is toxic; the concentration may only be high enough to damage the fish, or can be high enough to kill them. Thiosulfate alone is useless for this job, regardless of the dosage. Thiosulfate has no affect whatsoever on dissolved ammonia gas. Bummer! We must use newer and specialized agents which specify on the bottle that they do each and all of the three jobs required.

There are a number of commercial products which specify in print that they “destroy” (or other terms to that effect) chloramines. That is valid even if the only active agent is thiosulfate – it does break the chlorine-ammonia bond which defines chloramine, so technically the chloramine is no longer there. Does that mean the water so treated is safe to use? No, it definitely does not. The freed chlorine gas must be converted to chloride ion, but as with the bond breaking, thiosulfate can do that as well, and is cheap and safe – so double the chlorine-only dose and cover the freed chlorine as well. Is the water now safe to use in the tank? No, unfortunately not. It still has all the ammonia released floating around at hazardous levels. If the product does not specify that it locks the ammonia into the harmless ammonium ion form, or at least notes that it “neutralizes” both the chlorine and the ammonia released, we have to assume it does not do this – commercial products never claim less that they do. “Destroying” chloramine is required, but is not sufficient. This is a key point, do not be misled. Both of the freed dissolved gases must be “neutralized” to make the water safe. This is where the marketing wizards take advantage of the chemically and biologically naïve. You do have to both read and understand the fine print, or you could kill your fish. Strictly as an FYI, yes, I have killed fish that way. I will not do that again. Specialized agents are available which do the whole job – break the chloramine bond and convert both freed toxic gases to harmless ions. Unfortunately, this is another situation where you cannot trust your local fish store, nor the chains, or mail-order houses. They quite likely do not understand the chemistry themselves. You need to ask on-line for suggestions of brands which do all the necessary jobs reliably, or search the manufacturer’s site for detailed information – if they do not clearly state that all three tasks are done, that product is not suitable.

There is another complication with post-chloraminated water. It still reads positive for ammonia on most hobby test kits. Read the information on your test kit for ammonia. If it specifies that it reads “total ammonia nitrogen” (or TAN), you will see positives with your test after using a good anti-chloramines agent. These are not false positives. They are real and valid, but do not necessarily indicate a hazard to your fish – which the kit instructions historically have listed as hazardous. Remember that ammonium ion (NH4+) is harmless, only ammonia dissolved gas (NH3) is dangerous, just as was the case for chlorine gas versus the ion form. The effective anti-chloramine agents lock all free ammonia gas into the ammonium ion form – which is harmless. The problem is that our 20th century tests are no longer adequate in this century. There are tests available which read only free ammonia (NH3), but to me they are not yet user-friendly. Technology changes rapidly these days, hopefully more user-friendly but adequate test kits will available soon. Until then, we must use the proper dose of an effective agent and rely on it working, or prescreen with difficult-to-use tests.

For what it is worth, I use Seachem’s “Prime” for chloramines, and “Genesis” for chlorine-only.


1. http://en.wikipedia.org/wiki/Chlorination

2. http://en.wikipedia.org/wiki/Chloramine

3. http://www.epa.gov/ogwdw000/disinfectio … index.html

4. http://www.lenntech.com/processes/disin … lorine.htm

5. http://www.lenntech.com/processes/disin … amines.htm

RTR/Robert T. Ricketts, Jr.

Published by

Robert T. Ricketts

Retired research scientist (biochemistry and physiology, pharmaceutical development) and senior process analyst. Started fishkeeping in the dark ages (1950s), first SW tanks in the mid-60s, first puffers in the early 60s. Started with two tanks and never less than multi-tanked excepting some periods in college and grad school. Specialty if any would be filtration and water management. Primarily species tanks, planted whenever possible/practical and some where it not really practical. Ran something on the order of >150 tank-years* in studying optimum tank conditions for F-8 puffers, the largest tank study I have done. Other studies have been significantly less. Alternate canister use was mid-40s, OERFUG just over 60, veggie filters only about 25 to publication, but still going on less intently. If it had been known that the F-8s would live so long, it probably would not have been started at all. *One tank-year is one tank for one year.