Anybody have any experience using mechanically softened water to fill their tanks or when doing top-offs?
I use well water that runs through a softener. It comes out at KH about 18, pH at 8.2, and GH at zero.
It also has high TDS, about 650ppm out of the tap. In general, a toxic soup that no one would recommend.
However, somehow I manage to do O.K. Fish thrive, and I'm successful with certain plants. Notice I said certain. There are some I just can't grow, and some that flourish. It's really just a process of trial and error. Is it ideal? Probably not, but very doable.
That being said, I have considered for some time going with a RO water system, and just may someday. In the meantime, things overall are pretty darn good.
Now as to the adding of salt, this was very popular back in the 80's/90's. There was a theory that a bit of salt would create a thicker slime coat, and make fish more resistant to picking up disease. Could have been a bunch of hooey, but who knows. Sounds like you are hearing from some old timers (no offence, I'm one too) who have been doing it all these years. I used to know a guy pretty well who owned a LFS, and he put a little salt in all his fresh water tanks for decades.
Personally I think there are no real benefits, and these same folks would do as well or better without it. And I seriously doubt any of these folks have planted tanks. Salt and the planted tank are not good companions.
And remember to keep in mind that no two tanks are exactly the same. Results can and do vary, and no one can give you an absolute answer on what will or what won't work in your tank.
A large part of this hobby can only be learned by trial and error. Over time you will find out what works in YOUR tank, so let your experience be your guide.