Chlorine can be deadly to your fish. What most people don't realize is that drinking water often has small amounts of chlorine, which is safe for people, but toxic to fish. Always use a commercial de-chlorinating compound when adding new water to your aquarium. Simply leaving water "standing" by itself, without using something to remove the chlorine, does not always work. There may still be some toxic chlorine and chloramine left in the water.
For more information, please read the story The Perils of Chlorine.