Chlorine and chloramine: is there a difference?

In the past, all tap water traditionally contained chlorine, which is highly toxic to fish. To remove chlorine, just add a suitable treatment. But did you know that most tap water now contains a chemical called chloramine? So what is chloramine? In this complete guide we look at chlorine vs. chloramine and we explain the crucial difference between them that every fish farmer should know.

Most aquarists know that you should perform partial water changes every week to remove toxic ammonia, nitrites, and nitrates from the tank and keep the environment healthy and safe for your fish.

Uses of chlorine and chloramine

Under the Safe Drinking Water Act (SDWA) of 1974, the Environmental Protection Agency developed quality standards for all public supplies in the United States. Those regulations require that all municipal supplies be chemically treated to ensure they are free of bacteria and other potential contaminants, making the water safe for public consumption and use.

For many years, chlorine was the chemical disinfectant of choice used by water utilities to treat water. However, most public treatment plants have now switched to using chloramine.

To further complicate matters, most city water treatment companies do not treat the supply on an ongoing basis. Coliform levels are monitored and when a certain threshold is crossed, the system is effectively bombarded with chemicals.

Consequently, on some days the levels of chlorine and chloramine in the water will be very low, while on other days the levels will be much higher. Generally, chlorine/chloramine levels in tap water range from 0.5 to 2.0 ppm (parts per million).


Chlorine gas dissolves in tap water to kill harmful bacteria that can enter municipal supply pipes. Water containing chlorine can be made safe for fish tank use simply by letting the liquid sit for a day or so and shaking it from time to time. Dissolved chlorine returns to its gaseous form and evaporates, leaving the water chemical-free and safe for your fish tank.

That’s the method I used when I first got into fish farming in the ’80s. Chloramine, however, is a totally different animal.


Chloramine or monochlorine is used by water treatment companies because it is not as prone to evaporation as chlorine and is essentially a more stable and reliable chemical. However, chloramine contains chlorine and ammonia, which is very toxic to fish.

The bad news for aquarium owners is that the traditional “rest and shake” method of removing chlorine toxicity does not work with water that contains chloramine.

Adverse effects of chlorine and chloramine

Now, while you may not notice any difference in your tap water, chlorine and chloramine can have devastating consequences for your fish if you’re not aware of the dangers the chemicals present to your pets.

chemical burns

Even at relatively low levels, chlorine causes chemical burns in fish and invertebrates, affecting their gills and internal organs.

Fish suffering from chlorine poisoning become very stressed due to the continual pain caused by the burning. Some victims develop redness (hyperemia) in parts of the body, gasp at the surface as they struggle to breathe through swollen gills, and swim erratically due to neurological damage caused by the chemicals.

If chlorine or chloramine levels in the tank are high, fish get sick and die very quickly, often within hours. Smaller fish species are generally more susceptible than larger ones.

Causes of Chlorine and Chloramine Poisoning

Chlorine poisoning typically occurs when inexperienced hobbyists fail to treat tap water before adding it to their aquarium during routine tank cleaning and maintenance. Pond fish are sometimes poisoned when a homeowner fills their fish pond and then forgets to turn off their garden hose. Excessive amounts of untreated water enter the pond and the fish die.

Did you know that using sponges, filters, and nets that have been exposed to tap water can introduce harmful levels of chlorine and chloramine into your fish tank? That’s why you should always rinse these items with seasoned tank water before use, not tap water.

Do you have chlorine or chloramine in the water?

The easiest way to find out if your tap water contains chlorine or chloramine is to call your water supply company and ask what they use to treat household supplies in your area. Water companies are legally required to provide you with information about the composition of your water. However, chemical additives and water sources may change throughout the year, based on seasonal and environmental factors.

Alternatively, you can test tap water using a test kit that identifies the presence of chlorine and chloramine. If you have a standard ammonia test kit, you can use it to test your tap water. If chloramine is present, the supply will most likely test positive for ammonia. Once you know what is in the water, you can choose a suitable aquarium treatment product.

Removal of monochloramine from water

There are a variety of ways to remove chloramine:


The most effective and easiest way to remove chlorine and chloramine from tap water is to use a chemical dechlorinator. There is a wide range of these products that you will find on the shelves of your local fish or pet store. However, not all dechlorinators are formulated to combat chloramine, so read the label carefully before purchasing one.

Dechlorinators contain sodium thiosulfate which reacts with chlorine, turning it into harmless chloride ions and making tap water instantly safe for use in your aquarium. However, if the water contains chloramine, even though the chlorine is removed from the water, the ammonia part of the chemical is left behind, so the water is still toxic to fish. So always make sure the product you choose addresses chloramine and chlorine.

Ultraviolet light

If chloramine is a problem in the tap water in your area, you can install a special unit in your home as part of a dechlorination system. UV light can safely destroy chlorine and chloramine in water without the need for additional chemicals.

Ascorbic acid and sodium ascorbate

According to the US Department of Agriculture Forest Service, two forms of vitamin C, ascorbic acid and sodium ascorbate, are very effective chemical methods of neutralizing chlorine, and neither is harmful to aquatic life.

Both forms of vitamin C are stable and have a shelf life of up to a year if stored properly. That said, once diluted in water, both chemicals wear off within a day or two.

Activated carbon

Activated carbon can be useful for removing chlorine and chloramine from water through a catalytic reduction reaction. That chemical reaction uses the transfer of electrons from activated carbon to chlorine, reducing chlorine to a harmless, non-oxidative chloride ion. However, even if you have an activated carbon component in your aquarium filter media, you should still treat fresh tap water before adding it to your tank.

Campden Tablets

If you like to brew your own beer, you’ll be familiar with Campden tablets. Brewers use Campden tablets to sterilize beer, cider and wine and to inhibit the growth of wild yeast. The chlorine and chloramine contained in tap water can impart an unpleasant taste to the brew, and Campden tablets are used to remove these chemicals from the water.

To remove chloramine and chlorine from tap water, simply add one Campden tablet per 20 gallons of water, let stand for 20 minutes, and voila!

“Stand And Stir”

If your water contains only chlorine, you can adopt the traditional “stand and stir” dechlorination method.

Just fill a bucket with tap water and let it sit for 24 hours, preferably outdoors. During that time, stir the water vigorously. That action helps stir up the unstable, dissolved chlorine gas in the water so it evaporates, leaving you with a bucket of fish-friendly water.

However, that method does not work with chloramine because it is a more stable chemical.

Home Chloramine Filters

If chloramine levels are very high in your area and your tap water tastes unpleasant, you may want to invest in a home carbon filter system. These systems are installed on shower heads and faucets and work by removing chlorine from the chloramine molecule and then removing residual ammonia through a cation filter or reverse osmosis.

Although sink water filters are available, the best way to go is to install a whole house filtration system, but that can cost up to $15,000 and maintenance costs are typically around $1,200 each year.

In conclusion

Chlorine and chloramine used to disinfect tap water for human consumption can be lethal to fish. Therefore, you should treat your tap water before adding it to your aquarium during weekly partial water changes.

Before treating your water, check with your local municipal water company or test your tap water yourself using a chlorine test kit to determine if your water supply contains chlorine or chloramine, as some dechlorination products do not work with the chloramine. Check the product label on dechlorinators. to make sure the one you choose is right for your situation.

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