Since domestic water softeners soften water by replacing calcium and other minerals with sodium, the effects are similar to putting a goldfish in saltwater. ... This can kill the fish depending on how much sodium is in the water.

Is soft water bad for fish?

Softened water may not be suitable for exotic, ornamental or tropical fish because they can be affected by the minerals in either hard or soft water. ... You may need to dechlorinate tap water of any kind before adding it to your aquarium.

What fish do well in soft water?

In some cases soft water fish need very low pH values to do well. The classic examples are things like Pikeheads, Ram Cichlids, and Chocolate Gouramis.