"People" really only care about water hardness because Ca
binds to soap - making it less effective & forming soap scum. Since Na
does not do that, the water softener simply exchange the Ca
& they call that soft water.
Although the softener removed Ca
& exchange it for Na
, it does nothing to the negative ions (like HCO3
). So the water is still loaded with ions after treatment --- Osmolarity actually is increased as you need 2 Na
to exchange for 1 Ca
As I said before, aquatic life is concerned with osmolarity, & less so with the actual composition of the ions (within reason! ). So when we say a fish prefers hard or soft water, we really mean it prefer higher or lower osmolarity.
Personally, I think you can use water softerner water for fish, as long as you know its composition and compensate for it. In addition to lack of Ca
- which fish & esp. inverts need, other trace elements are removed & that might be important. Another problem is that people think the treated water is same as R/O or "pure water", and the water chemistry will get really out of wack if you add stuff based on that erronous assumption.