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The threat of chlorine poisoning from tap water has always been with the urban Koi keeper. However, due to the demands of an ever-increasing population, water is now delivered with a more potent disinfecting dose.

Beginners to the hobby of Koi keeping tend to think that since tap water is fine for humans, it should be equally fine for their fish collection too. To their dismay, they often discover that it is not the case when their Koi suddenly becomes ill or even die, after a major water change. Chlorine poisoning is the most likely cause. With time, the more experienced Koi keepers learn how to cope with the chlorine threat, but now, due to the demands of an ever-increasing population in urban areas, water from the tap comes with a far more potent disinfecting dose.

It is the responsibility of water supply authorities to ensure that all water delivered is safe for its intended domestic use. Chlorine gas is mostly used for this purpose. The chlorine gas combines with the water to form hypochlorous acid (HOCl) that is one of the most effective antibacterial agent known to man. Above pH 6,0 hypochlorous acid dissociates into hypochloride ions (OCl-) which is about 100 times less effective at disinfecting. At pH 8,0 all the chlorine is in this form. The availability of these three chemicals (chlorine gas, hypochlorous acid and hypochloride) in the water is termed the free chlorine residual.

Water supply authorities like the Rand Water Board aims 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, in the process it dissipates quickly leaving the more distant delivery points of the network potentially unsafe. In order to ensure these minimum health requirements, treatment plants deliver water into the network with as much as 10 mg/l free chlorine. 

This is a costly exercise and water supply authorities are now using chloramine in combination with chlorine as a disinfectant. Chloramine (pronounced -mean) is an inorganic complex that is formed when ammonia combines with chlorine in water. Unlike chlorine, chloramine 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. Koi keepers now do not only have to contend with chlorine, but also with the extended potency of the chloramine and additional ammonia, when they fill their ponds.

Chlorine in the water triggers an escape response in the fish, and since they cannot escape the confines of the pond, their stress level increases to a point where they become exhausted and weak. They will finally stop ventilating because of the destructive effect the chlorine has on their gill tissue, hang motionless at the surface and die.

You may have witnessed this behaviour when Koi are released into new environments like in show display ponds if the water was not adequately detoxified beforehand. The fish will shoal and swim energetically around the pond. Your first reaction will be: Hey, look at this fish! They sure are happy and alive. However, donít be fooled. Soon they will start slowing down, and the next morning, when the show opens, and the public rushes in, they will be either dead or looking miserable and decidedly unhealthy.

Toxicity levels of chlorine are usually quoted as combined residual chlorine, which represents the total free chlorine and chloramine content of the water. A combined residual chlorine concentration of 0,3 mg/l is sufficient to kill most fish fairly quickly, while 24 hour exposure to levels as low as 0,04 mg/l can prove fatal. Therefore, any measurable levels of chlorine residuals should be regarded as unsafe for fish in the long term.

When a pond is filled up with fresh tap water the fish can be exposed to high chlorine levels for many days. The possibility of increased ammonia levels should also not be ruled out. The chlorine could kill sufficient bacteria in the filter, which can make it impossible for the pond system to cope with the increased ammonia level. Chlorine and ammonia levels can also vary considerably from time to time and from area to area, making it difficult to adhere to a standard modus operandi when filling ponds. (The pH of the tap water of a specific site have been seen to vary between 7,1 and 8,7 within 5 days) Koi keepers should invest in a chlorine test kit that will indicate the total chlorine. Kits used to test the chlorine of swimming pool water are not adequate because they measure free chlorine residuals only.

Tap water typically contains 0,1 mg/l ammonia as a result of the chloramine manufacture process. This is not too severe, but there is no guarantee that it will not be higher sometimes. Koi keepers should therefore test the ammonia level of their tap water as well before filling the pond. They might well decide to postpone their water change for a few days!

Various proprietary products are available from pet shops and Koi dealers that will neutralise chlorine. Sodium thiosulphate, also known as hypo or fixer, is readily available from chemists, co-ops or photographic shops in a crystalline (large monoclinic crystals) or anhydrous (white power) form. An application of 7 mg/l sodium thiosulphate in the crystalline form is required to neutralise 1 mg/l of chlorine (5,5 mg/l sodium thiosulphate of the anhydrous form will do the same).

Pond keepers are often told that they should de-chlorinate tap water before putting it into their pond, While this is ideal, it is often impractical and not really essential if the exposure to the chlorine is short-termed. The following procedure for doing water changes is both safe and simple:

To ensure some degree of dilution of the chlorine in the water one should never do more than a 50% water change. Calculate the volume of water to be replaced. If you do not have a residual chlorine test kit handy, assume that the tap water contains 2 mg/l  chlorine. Dissolve in a bucket 7g sodium thiosulphate (one heaped teaspoon) for every mg/l chlorine and for every 1000 litres of pond water treated. (A 10,000 litre pond will require about 10 X 2 X 7 X 0,5 = 70 gram hypo for a 50% water change.) Fill the pond until it is almost full and then slowly add the thiosulphate to the pond while using the water jet to create a current that will ensure proper mixing.  

But if this is all a bit technical for you, take a walk to your nearest Koi or pet shop and ask for their advise for some off-the-shelf remedy to neutralise chlorine in pond water. Do make sure you buy enough for the volume of your pond. Do make sure you follow the instructions diligently.

Small water changes of less than 10% should not worry the Koi keeper. The organic matter in the pond water will greatly assist in de-chlorinating the free chlorine while the dilution effect will render the chloramines harmless. If thiosulphate is not used, the water should be sprayed into the pond in a fine mist in full exposure of the sun to assist in the decomposition of chlorine. Chloramine and ammonia will not be affected by this treatment though and special precaution must be taken for it.

As our urban population grows, Koi keepers can expect to have to deal with a deteriorating water quality from their taps. Who knows what the future has in store, but judging from the battle Koi keepers in Japan and the UK have, we should still be quite thankful for tap water that we can both drink AND keep our Koi in.

A revised version on an article first published in KOISA, newsletter of the South African Koi Keepers Society, December 1995, by Servaas de Kock  


Photo: ZNA

A show quality Koi of the Tancho variety. Needless to explain why the Japanese place them in such high regard.

Even so: a very difficult fish to breed and raise to perfection.

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Copyright © 2004-2009 Servaas de Kock
Last modified: 27 September, 2005