Rainwater p.H 7.2 & low salt, Is it OK for goldfish?

We live in an area where the tap water is horrible and rely on
rainwater for household use. We don't have industrial polution
affecting the rainwater quality and lttle chance of agricultural
contaminants! If we buy goldfish from town where the filtered
tapwater has approx 500mg salt per litre (plus chlorination and
everything else) obviously we cant just transfer them to straight
rainwater so we add a "rainwater conditioner" which is a salt
solution. Can we gradually dilute the solution by adding straight
rainfater with each change (25%) of tank water? Do goldfish have a
requirement for some salt (can they live in crystal clear mountain
streams)? It would be an advantage as far as reducing algal growth if
the water was purer.