The short answer
Yes — tap water is what most aquarists use for water changes. The two things you must do every time are dechlorinate it and match the temperature to the tank. Get those right and tap water is perfectly safe. Skip the dechlorinator and the chlorine will harm your fish and damage the beneficial bacteria in your filter.
Always dechlorinate
Mains tap water is treated with chlorine or chloramine to keep it safe to drink — and both are toxic to fish and to the bacteria that keep your tank cycled. A water conditioner neutralises them instantly, so add it every single time you use tap water. Dose for the volume you’re adding, following the bottle’s instructions.
Chloramine matters here: unlike plain chlorine, it won’t gas off if you leave water standing, so a conditioner is the reliable way to deal with it.
Always match the temperature
Fish are sensitive to sudden temperature swings, which stress them and can trigger illness. Before adding new water, get it close to the tank’s temperature — for most people that means mixing hot and cold from the tap to roughly the right warmth, or letting treated water sit until it matches. See how to match water temperature for the practical methods.
When tap water needs more care
In some areas tap water carries ammonia (from chloramine) or is very hard. A good conditioner detoxifies ammonia and chloramine as well as chlorine — see removing ammonia from tap water. For sensitive species like shrimp you may want softer water; see lowering water hardness for shrimp. Browse our conditioner picks, check your source with a test kit, and follow how to do a water change.