First I would check if they have been coated with something eg lacquer. If they haven't they are fish safe, as long as you remove any traces of dust, candle wax residue, aerosol residue etc - and detergent if that's what you used to wash them
Shells will dissolve slowly if the pH is below 7.0. This will increase hardness (GH) carbonate hardness (KH) and pH slightly. But if your pH is above 7.0 they should hardly dissolve at all.
You have soft water but your pH is not necessarily below 7. My hardness is 5 German degrees but my pH is about 7.5.
