Tap water is the most recommended aquarium water of choice for freshwater fish. It is also cheaper than RO water.
Can you treat water with fish in it?
Most people use tap water in their tanks; it is easiest (and cheapest) to use. Unfortunately (for aquarists), local water companies add chemicals to the water to make it safe to drink (e.g., by adding chlorine or chloramine to kill bacteria).
Can I use tap water in my fish tank?
It is recommended that purified mineral water be used instead of distilled water. Alternatively, tap water can be used, but it must be treated to remove chemicals that are potentially toxic to fish. Chlorine or chloramine is often added to tap water as a purifier, but such chemicals can damage the gills of fish.