Chlorine/Chloramine Toxicity

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Chlorine is added to tap water to kill microorganisms so that it is safe for human consumption.

Sometimes ammonia is also added, which reacts with chlorine and produces a stable disinfectant, chloramine. Chloramine is less likely to react with organic matter and does not form derivatives which are toxic to humans.

Both chlorine and chloramine are highly toxic to all fish, and although toxicity depends on a variety of factors, any measurable amount is considered extremely harmful.

Most aquarists are aware of the dangers of chlorine-containing tap water and the disease is uncommon. Problems most commonly arise when there is a failure to treat for chlorine in municipal tap water.

Chlorine is also used to treat industrial effluents and therefore fisheries should not be situated close to chlorinated effluents.

Clinical Signs

Chlorine and chloramine both cause acute necrosis of the gills.

Clinical signs include piping, dyspnoea and acute to subacute mortality. Chronic exposure generally results in extensive mucus secretion and hypertrophy of the gill epithelium.


Presumptive diagnosis can usually be made from the history.

This usually involves fish having been added to a tank within days of setting up the tank, tap water used to top up a tank, recent water change in an established tank, an unsupervised hose left running when topping up a tank, unrinsed chlorinated utensils used in the tank.

It is important to determine if fish were exposed to chlorine or chloramine-treated water, and this can be determined by contacting the municipality.

Commercial test kits for chlorine and chloramine are available to accurately determine levels in the water, and any detectable amount of chlorine or chloramine is undesirable.


For emergency treatment of chlorine or chloramine toxicity, fish should be immediately transferred to chlorine-free water. The water temperature should be reduced to increase the level of dissolved oxygen, and vigorous aeration and supersaturation with oxygen should be performed where possible.

Parenteral administration of dexamethasone may possibly improve the prognosis.


Chlorine is easily removed from water by vigorous aeration for 24 hours or by adding commercial dechlorinating agents. Chloramine cannot be removed by aeration and a chemical neutraliser is needed to break the chlorine-ammonia bond, which means the free ammonia then also needs to be removed.

Sodium thiosulfate is commonly used to dechlorinate water and is an effective and safe compound.

Chlorine/Chloramine Toxicity Learning Resources
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Ornamental Fish Q&A 06


Detrich, H.W. (2004) The Zebrafish: genetics, genomics, and informatics Academic Press

Roberts, H. (2009) Fundamentals of Ornamental Fish Health John Wiley and Sons

Noga, E. (2010) Fish Disease: Diagnosis and Treatment John Wiley and Sons

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