Chlorine and Chloramine

Chlorine and Chloramine

Chlorine is toxic to our fish and we need to understand the risk when using municipal treated water.

Chlorine is a very effective, widely used disinfectant. In order to protect the consumer, water supply authorities commonly use hypochlorite compounds or chlorine gas for disinfecting the water to ensure that all water delivered to households, is bacteria free. Treatment plants attempt to deliver water to household taps with a 0,3 to 0,8 mg/l free chlorine content. However, while chlorine is a very effective disinfectant, it dissipates quickly leaving the more distant delivery points at risk of unsafe water. This prompts treatment plants to deliver water into the network with as much as 10 mg/l chlorine.

Because this is a costly water treatment, treatment plants combine chlorine and ammonia to form the complex chloramine, which is effective as a disinfectant for longer at a lower concentration. Chloramine at this concentration is harmless to humans but deadly for fish,36,37 and in contrast to chlorine, is a very stable compound that will remain active for many days even after being aerated or exposed to direct sunlight. When it does break down, it dissociates into free chlorine and ammonia. The ammonia is normally taken care of by the biological filter but is a point to consider in the absence of filtration.

Chlorine is harmful to fish because of the destructive effect it has on gill tissue. Toxicity levels of chlorine is usually quoted as combined residual chlorine which represents the total free chlorine and chloramine content of the water. A concentration of 0,3 mg/l combined residual chlorine is sufficient to kill most fish fairly rapidly while long-term exposure to chlorine levels as low as 0,003 mg/l will prove harmful.38, 39, 40, 41 The kits used for swimming pools that measures free chlorine only, will therefore not suffice and can be misleading.

Koi keepers using municipal tap water should be aware of the chlorine and chloramine content of the water and take appropriate precautionary action. Various proprietary products are available from koi outlets to neutralise chlorine in pond water. Some of these products like Chloram-X, will not only remove chlorine but also chloramines. Sodium thiosulfate is most commonly used and readily available in crystal or powder form by a variety of common names like AntiClor Xtals etc. Apply sodium thiosulfate (crystalline form) at 7 grams per 1000 litre to neutralise 1 mg/l of chlorine.

Sodium thiosulfate is relevantly harmless to fish and pond keepers can safely increase the dosage two or three times if they think the chlorine concentration is more than 1 mg/l. Koi keepers should consider investing in a chlorine test kit that measures total chlorine for safety.

The safest and easiest way to limit the chlorine hazard is never to replace more than 20% of the pond water with tap water, to spray the tap water onto the surface and only when the pond is exposed to direct sunlight. Treating with a dichlorination product as a standard best practice procedure. Please pay due attention when topping up with tap water. Many a pond keeper found to his horror belly-up fish when forgetting to turn off the tap in time. Immediate dichlorination while vigorous aerating is called for to rescue the collection in such a case.

(Extracted from Living Jewels (1996) by Ronnie Watt and Servaas de Kock. Updated in 2007 and later.)

Reference material sited:
36. McDowall, A. (Editor), 1989
The Practical Encyclopedia of Koi
Salamander Books Ltd.

37. Anonymous, 1991
Chloramine in Tap Water is Poisonous to Tropical Fish
Aquarium Pharmaceutical Reference Sheet API

38. Pillay, T.V.R., 1990
Aquaculture, Principles and Practises
Fishing News Books

39. Boyd, C.E., 1990
Water Quality in Ponds for Aquaculture
Birmingham Publishing Co.

40. Marjanovic, N. and Orlob, G.T. in Lannan, J.E.; Smitherman, R.O. and Tchobanoglous, G. (Editors), 1986
Principles and Practises of Pond Aquaculture
Oregon State University Press

41. Anonymous, 1991
Chloramine in Tap Water is Poisonous to Tropical Fish
Aquarium Pharmaceutical Reference Sheet API