The plumber who installed my rainwater tank came back to replace the filter on my mains water and I told him I had had to stop drinking the rainwater as it had turned me acidic (yes it really did, three days after I stopped drinking it I was in bliss) and he told me it was the deteriorating filter that made that happen so he replaced the filter on the rainwater tank too.  

So yes, not something to worry about unless you have filters.  Or acid rain of course.

I'm very pleased with myself that I thought to get the test kit out and I've taken to mixing the tap water with the rain water.