One of your koi-owning clients contacts you early one morning and she is very distressed. All nine of her prized koi are dead and floating in the pond. They were apparently normal last night and had good appetites for their evening feeding.
The 2,500-L pond is supplied with city water and has adequate biological and mechanical filtration as well as an aerating fountain. All components are functioning normally but the owner mentions that the person who ‘topped off’ the pond last night with water had inadvertently left the hose running all night (the hose can be seen in the photograph).
| Question | Answer | Article | |
| What is at the top of your differential list? | Chlorine/chloramine toxicity |
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| How can you confirm your diagnosis? | By testing the water for chlorine levels. In this particular case, levels were recorded at 1.2 p.p.m. (several hours after the water sample was obtained). A bench-top chlorine titrimeter was used, although much simpler and less expensive colormetric tests are available. |
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| What are the principles of chlorine toxicity? | Chlorine reacts with living tissues causing acute necrosis. Because the gills are vulnerable and exposed directly to the aquatic environment, this can lead to respiratory difficulty and asphyxiation. Fish experiencing chlorine toxicity will appear very stressed. Morbidity and mortality depends on chlorine levels in the water. High levels (greater than 1.0 p.p.m.) may cause fish to succumb in hours or even minutes. Affected fish may be piping at the surface, swimming abnormally, and they may appear pale and mucus covered. Most municipal water has been chlorinated to disinfect it for safe human consumption. Although relatively harmless to humans, chlorine can be deadly to fish. The amount of chlorine in tap water may fluctuate but is usually between 0.5 and 2.0 p.p.m. Chlorine can be ‘bubbled’ out of water by aerating for several days in a container with a large surface area. Another commonly used disinfectant is chloramine. This compound combines chlorine with ammonia, both of which are harmful to ornamental fish. Unlike chlorine, chloramine does not produce trihalomethanes, which are toxic to humans. |
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| What other conditions resemble chlorine toxicity? | Many toxic conditions will resemble chlorine poisoning (ammonia, copper, and organophosphates). An accurate history will usually rule these out. Hypoxia caused by overcrowding or poor aeration can also look like chlorine toxicity. |
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| How can such a problem be prevented in the future? | In cases where the fish are still alive, the contaminated water must be immediately neutralized or the fish removed to clean, chlorine-free water. A number of commercially available compounds quickly and safely remove chlorine from water. These products usually contain sodium thiosulfate, which inactivates chlorine through a chemical reaction in which sodium chloride is formed. Sodium thiosulfate is inexpensive, effective, and safe (just 7 g of sodium thiosulfate will remove the chlorine from 1,000 L of municipal water with chlorine concentrations as high as 2.0 p.p.m.). After the chlorine has been removed, the water containing the fish should be aerated well with room air or preferably 100% oxygen. Temperate species like goldfish and koi will benefit from reducing the water temperature to increase dissolved oxygen levels. When possible or practical, administering dexamethasone intravenously or intraperitoneally at a dose of 2.0 mg/kg every 12 hours may improve the prognosis. |
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