Well, yeah, I have one of these soft water systems, and I recently started to keep up with replenishing the salt in the reservior. Sometimes I neglect replenishing the salt in the system, and the water softener just stops working. When that happens, my tap water reverts back to being very hard, well over 450ppm GH hardness.
There are times that I am neglectful at replenishing the salt, and there are times that I've kept up with it. I just use tap water for water changes and top offs, sometimes softened, and sometimes not. I've never noticed any adverse effect from the tap water in the past, softened or not.
But now I've added a couple of German Blue Rams, and I wonder if over time, they might be adverse to it. I've had some glow fish, and corydoras over the last few years, and they seem fine either way with my tap water, softened or not.
Since I started keeping up with the salt replenishment lately, it occurred to me to check into whether the softener might be adversely affecting the tank water when I do water changes and top-offs.
So, I simply searched various internet forums to see what people had to say. A surprising number of fish keepers said they dose a very small measure of salt to their fresh water aquariums when they do water changes. And, they weren't doing it as medications, but as a routine practice during water changes. If you Google it, you'll see what I mean. And, some of them are old timers too, with many years of experience in the hobby. In fact, they swear by it!
I know what you mean by your skepticism, and I'm not advocating it. But, some of them claimed that they've been intentionally doing it for 30 years with no adverse effects on their fish, and that they believe it results in positive effects in doing so.
Plants? Well, that might be a different thing. But, some of them did claim to have plants, and they claimed to have experienced no adverse effects. I'm sure it depends on the type of plants one has, assuming that what they reported is true.
There were also numbers of fish keepers that advised against adding small amounts of salt to a tank, and against using water from a soft water system, claiming that the residue from the salt would be detrimental to the tank.
Personally, I can't see it, and especially if what some of the fish keepers that use small amounts of salt in their tanks report, but also because you can't even taste the salt when I draw softened water from my tap.
But, because I find that members in this forum seem to be generally more informed compared to some other forums, I am curious what opinions the members of this forum may have with regard to using softened tap water where salt is the cleaning agent used by the system to keep the resin collector clean.
Who are these people that add salt to their freshwater tanks on a regular basis? Freshwater fish (unless brackish) do not like salt. Freshwater plants do not like salt. What benefits would this provide? The only times I hear people add salt to their freshwater tanks is as medication to combat stuff like parasites.
Having said that I have no idea about mechanically softened water / if it could be harmful. Out of curiosity are you looking to soften your water in the first place?