you don't want to use softened water for your plants It's just hard water filtered through salt to remove the calcium that builds up on shower doors, etc. Hard water is better for your plants, that calcium is a good thing.
What's the EC of your tapwater? If you don't have a meter, you can probably look it up for your locale online. If it's really too hard, you'd want an R/O filter. But once you start using RO, then you need to buy cal/mag supplements to replace the calcium you just removed....