Saw your question Rae and thought I might simplify what the softened water is like so you could decide.
-Dissolved "hard water" leaves scale which is lots of magnesium, calcium, and other metals that the groundwater picks up from the surrounding soil and rock at the source (if your city has hard water then they are leaving it to the consumer to soften, some do it at the plant, thats why at the end of this everyone would see that unless you know exactly what is going on at your water plant, different cities water is all different even if meeting general safe standards). Your house has softener, i have a well tapped into a spring aquifer below my lot so i am lucky, dont need softener as most on wells do, but from what i have learned about frogs skin i am going to go bottled cuz there is really no way to know whats in it. In the wild animals move away from water sources not appropriate for them.
-There is minuet salt in softened water but the softeners all vary in function
- The beads in the machine do the softening. They are negatively charged ions, the salt you put in is sodium ions which have a positive charge. The hard "metals" calcium, mag, etc are positively charged too. when the water flows through the beads (-) they swap with the sodium bonded to the beads (+) because they are an even stronger neg charge than the Na. So the beads grab the metals because Na switches place with the metals.
- Then even if that was ok water for the frogs, the machine has to rejuvenate when the beads get loaded with metals. the beads in this cycle are soaked in strong salt water and it breaks the metals bond and then the machine flushes this out and it starts all over. Problem is it is always going from soft to hard between cycles of rejuvination, either on a timer or some expensive ones that moniter metal build up again when the beads (-) cant hold anymore metals (+).
So for the frogs there are three issues:
1) the levels of metals vs salt is always fluctuating
2)The metals are being removed completely at some point in cycle for them kinda like distilling
3) is the salt even though safe for us ok to be absorbed in there skin constantly---lakes rivers stream humidity and rain are not filled with our salt rock at all, or atleast at minute levels they can decide to move away.

Sorry so long but atleast you might understand and decide, i would think its the same problem for them that we have, freshwater is hard to find.