Rainwater

I live in rural Australia and our water supply is rainwater which we collect
into large tanks. This is not a polluted area and the water is of very good
quality.

I have read in several places that rainwater is too pure to use unmodified
in an aquarium because it "lacks minerals essential for life" and I have
therefore always hardened it up a little with a locally manufactured
product.

Is it necessary to modify rainwater when keeping sof****er loving fish such
as tetras (which I am now doing) or do I need to continue to add some
minerals?

Keith