Will softened water harm tropical fish?

If you have an aquarium, you may wonder about using softened water in the tank: for most tropical fish, soft water is fine. However, you'll want to make the transition gradually: start by exchanging about one-fourth of the tank water for soft water. Repeat the process periodically, over a number of days. After a while, the tank water will be completely converted to soft water. Making the change slowly gives the fish time to adapt to their new environment. As mentioned, most fish are perfectly adaptable to soft water. But if you're in doubt, you might check with a pet shop or aquarium store before you start. They can advise you as to the ideal conditions for your particular type of fish.