Most municipal water companies sterilize their water with chlorine or chloramine, a combination of chlorine and ammonia, for safe human consumption. While relatively harmless to humans in minute amounts, chlorine can be deadly to fish.
The amount of chlorine in tap water may fluctuate, but it is usually between 0.5 and 2.0 parts per million (ppm). Chlorine in water reacts with living tissues and organic matter causing cell death (acute necrosis) in fish. Since fish gills are sensitive and exposed directly to the aquatic environment, the death of cells in the gills - the fish's breathing apparatus - can lead to respiratory difficulty and asphyxiation.
Chlorine poisoning is usually caused by inexperienced fish hobbyists either putting fish in unconditioned common tapwater, or replacing a large amount of tank water with unconditioned chlorinated water. One common scenario is when a koi or goldfish pond owner "tops off" his pond and forgets to turn off the garden hose. Too much unconditioned water gets in the pond and the fish die. Even unrinsed chlorine sterilized utensils (nets, sponges, filters) can kill.
Chlorine Can Kill Fish in Minutes
Chlorine poisoned fish appear very stressed. How quickly they get sick and die depends on the level of chlorine in the water. High levels may cause fish to succumb in hours or even minutes. Generally speaking, smaller fish are more susceptible than larger fish.
Affected fish may appear pale and covered in mucus. Some will exhibit redness (hyperemia) on various parts of their body. Fish may be piping at the surface for air and swimming erratically.
Chlorine can be "bubbled" out of water if the water is well aerated for several days in a container with a large surface area. Chloramine, however, is more stable in water than chlorine, and cannot be bubbled away. Chloramine has become much more popular than chlorine in public water supplies because, unlike straight chlorine, it does not produce trihalomethanes toxic to humans.
Many toxic conditions will resemble chlorine poisoning (ammonia, copper, organophosphate poisoning). Hypoxia due to overcrowding or poor aeration can also mimic chlorine toxicity. Once these other causes are ruled out, it's time to treat the chlorine problem.
A number of manufacturers make chlorine test kits. Sophisticated water testing laboratories have a device called a chlorine titrimeter. In most cases the history and clinical signs alone can diagnose chlorine toxicity.
Taking Care of the Problem
Severely affected fish usually die. Fish that are quickly removed from contaminated water may survive if they are not showing signs of respiratory distress within three to six hours following exposure. To try to fix the situation, the contaminated water must be immediately neutralized, or the fish must be removed to an aquarium or other vessel containing clean, chlorine-free water. A number of commercially available compounds quickly and safely remove chlorine from water.
These products frequently contain sodium thiosulfate, which inactivates chlorine through a chemical reaction in which sodium chloride is formed. Sodium thiosulfate is inexpensive, effective and safe (just 10 grams of sodium thiosulfate will remove the chlorine from 1,000 liters of municipal water with chlorine concentrations as high as 2.0 ppm). After the chlorine has been removed, the water containing the fish should be aerated well with room air or preferably 100% oxygen. Temperate (mid-temperature) species like goldfish and koi will benefit from reducing the water temperature to increase the dissolved oxygen levels.
Since chlorine poisoning is almost always an acute problem, follow-up consists primarily of supportive care for the sick fish and prevention -keeping chlorinated water away from the fish.