Much of this water is lost by evaporation. This leaves the sodium salts in the soil. After successive waterings, there may be enough sodium salts to retard the growth of plants. A bypass or separate line carrying raw or hard water should be provided for all outdoor sillcocks. The same water should be used for watering indoor plants.
Soft water is satisfactory for most aquarium fish.
Softened water does not have a toxic or even undesirable effect on them. You should, however, change the aquarium water gradually. Replace about one-fourth of the tank water at regular intervals. Eventually the aquarium will contain only soft water. The tropical fish should be just fine.