I used softened water for my GF for years with no problem. I added some organic
dolomitic limestone to the water. I drank softened water for years. I have low
blood pressure. Ingrid

I think that using Softened water for any kind of fish is not a great idea.
Its very much too soft & depleted & then again much too full of Sodium ions
to be good for them (which is why we humans shouldn't drink it either!).