Almost all city municipalities treat their water systems with chlorine or chloramine to make it safe for people to drink. Chlorine is a cheap and efficient way to treat water and the levels used have no harmful effects on humans or fuzzy pets and animals. However, chlorine is extremely toxic to fish.
City water systems are treated with chlorine or chloramine. Chloramine is a combination of chlorine and ammonia, a more stable chemical that does not off gas like chlorine. However, both ammonia and chlorine are toxic to fish. Most city systems are not treated continuously, but when a certain threshold of coliforms is reached, the system is nuked. This leads to water having very low levels of chlorine/chloramine some days, but very high levels at other times.
Effects of Chlorine Toxicity
Fish exposed to chlorine can show signs of hypoxia, gill tissue necrosis and neurologic signs, including trouble swimming, incorrect body positioning and sudden death.
How are exposed fish treated?
There is little treatment for chlorine toxicity other than supportive care. Once the toxicity has been determined, conditioners should be added immediately to remove the chlorine from the water.
How do I protect my fish?
Add a water conditioner that treats chlorine AND chloramine after every water change. Small water changes (<5%), you might be able to get away without adding a dechlorinator, but its better safe than sorry.