I see. Well, I know I am new to the forum and all of that but I've been using softened water for years with no ill effects. I even keep and breed cichlids in it. IMVHO, "salts" is a very very big subject and I'm sure that there is some sort fo salt in every natural body of water. In fact, the definition of "freshwater", in most cases, is not that it contains no salt, only very low levels of salt...which is the case with softened water. I think the more important thing to consider is that you are able to maintain situation where your fish are acclimated to what you are using and do your best to avoid making any large, rapid, and/or frequent changes in water chemistry.
Besides, if you use RO water, you would want to replace the trace elements in the right quantities by using a product like Kent's RO Right...which contains "major salts of sodium, magnesium, calcium, and potassium".