Setting up an aquarium seems like simplicity itself. Just fill it with water and toss in fish, right? It isn't quite that easy, but it isn't rocket science either, as long as you know what the potential pitfalls are. One of those pitfalls is trying to use untreated water.
Water from our tap contains chlorine as a disinfectant. It keeps the water safe for humans to drink. At the same time, chlorine in the water irritates gill tissue. With enough exposure, chlorine is toxic to fish. That means that it must be removed or neutralized before the water can be used in a fish tank, and that applies equally to a new tank or a partial water change in an established tank.
Chlorine in drinking water occurs in two forms. Chlorine itself is not difficult to remove. That happens naturally as the water sits out for 24 hours. Easier, tough, is an additive to neutralize the chlorine.
Sometimes cities add ammonia to drinking water to help with the sterilization process. When chlorine and ammonia mingle in treated drinking water, chloramines can form. Chloramines can only be neutralized, not removed.
There are a number of products to neutralize chlorine and chloramines on the market. Any pet store that sells tropical fish supplies will carry these. More neutralizer is required to treat water containing chloramines than for water with simple chlorine. Check your city's annual water report to learn whether the water is treated with ammonia and chlorine, or only chlorine.
Metals are another contaminant in city water. Copper is especially toxic to fish. The good news is that many water treatment chemicals that neutralize chlorine are also useful to neutralize metals like copper. Read the label thoroughly before you buy. It won't hurt to treat something that isn't there, but it will hurt if you don't treat for something that is.