Indoor plant experts agree that it's best to avoid salt-softened water for houseplants. The level of salt in the water can vary drastically, depending on how hard your water is before treatment. Because you don't know the level of salt, it's difficult to make corrections for the damage. Because the soil for house plants is very limited, salt deposits can build up quickly, damaging the roots.
There is a possibility that the salt buildup from using water from a water softener in your garden can damage the soil by changing the texture. Gardeners suggest several remedies for this. First, it may not be an issue at all. Most water softeners are connected to indoor plumbing only, so your outdoor plumbing may bypass the unit altogether. However, if the outdoor plumbing is connected, you can turn the softener off during watering or add gypsum to the soil to counteract the chemical buildup of salt.
Because they are so large, it's difficult to think of trees as being damaged by the small amount of salt in softened water. However, trees are commonly damaged by salts in the soil and also on the needles of evergreens. There is a debate, though, whether the amounts in salt-softened water can have an effect over time. If you are worried about long-term effects, turn off the softener while you're watering.
There is evidence that salt in the soil will negatively effect the cells of a plant. The result for the rest of the plant is yellowing leaves and small flowers, if any. Although the amount of salt in softened water is slight, the buildup can impact the health and beauty of flowering plants.
Whether you have outdoor aquatic plants in a water feature or inside in an aquarium, experts suggest not to use salt-softened water in aquariums for a variety of reasons. Not only can the level of salt in the water harm some fish, it can also damage aquatic pants. The softer water can be beneficial for some types of fish and plants, but the presence of the salt can upset the delicate balance needed for aquariums and decorative ponds.