I assume so.... I am actually asking on behalf of a beginner, not myself.
If it's a salt based softener, then I wouldn't. The reason is because the softener replaces calcium and magnesium ions with sodium ions.
I'll pass this on to him. Thank you!
I have a salt-based water softener, so as AquaticJ said, the magnesium and calcium ions are replaced by sodium ions, but mine will also filter out the salt afterwards. I still don't use softened water (I just bypass the water softener when doing water changes) because the fish need the minerals that have been removed.