Can I use softened water in my aquarium?

Softened water is satisfactory for most fish and should not have any undesirable or toxic affect on the aquarium. When changing the water, it is always advisable to do so gradually. This is important so as not to disturb or unbalance the aquarium's environment. Temperature and pH of the water is critical to some tropical fish and any drastic change could result in fatalities. The change from hard water to softened should be undertaken over a month or so, replacing only about 25% of the aquarium's capacity at any one time.