If you've ever inhaled a lungful of pollutants while driving behind a large truck or bus, then you can imagine life in an unhealthy aquarium. Water quality problems can cause disease and even death, so monitoring your tank's water is the most important step you can take to keep your fish happy and healthy.
Your tap water contains chlorine or chloramines to kill such bacterial contaminants as E. coli. But these chemicals are deadly to fish, and you must remove them before turning your finny friends loose in their tank.
Air It Out
Chlorine will evaporate from standing water in two to three days; unfortunately, this is not true for chloramine. A chemical treatment must be use to remove chloramine from the water. A test kit will tell you when the chemicals are gone. You can remove both chlorine and chloramines immediately with a commercial dechlorinating product. However, dechlorinating products will remove the chlorine portion of chloramines that result in the release of ammonia so make sure the product you use also removes ammonia. Use extra caution when setting up a new tank, because inactivated chloramines (though not chlorine) can release ammonia. Tanks with established filtration systems can quickly convert ammonia to a harmless nitrate molecule, but new tanks can't handle the workload.
Even the limited chlorine exposure from adding small amounts of tap water to an established aquarium on a long-term basis can damage gill tissue and lead to death.
Any detectable level of chlorine is too much. If you detect chlorine in your tank water, immediately treat your water as appropriate and reevaluate your aquarium maintenance routine.