Your local water authority must send you a yearly water quality report where it would list almost anything they can detect in the water they supply.
I am on well water and run everything, both home and business water, through an RO. My system has the capacity to produce 10,000+ GPD of RO water of which I have been using 3,000-6000 daily for the past 4 years (continuous 24/7 water change in all my tanks and ponds).
So the sequence is:
well (~1500 ppm TDS (low end brackish), ~600 ppm out of the TDS is hardness, rest must be salinity, 1/4-1/2 ppm ammonia, tannin (yellow), sulfites (strong sulfur smell)) -->
sediment filter -->
RO membranes (~30 ppm TDS) -->
charcoal + calcite filter (calcite must have been spent a long time ago though) -->
another charcoal filter -->
added directly into four sumps for my 12 tanks (110 ppm TDS, 1-2 drops KH, same GH).
Altogether ~10-15 five-gal pails of seashells (thank you Naples beaches) are added to the sumps to gain a bit a hardness and minerals back (TDS raised to ~150 ppm, 2-3 drops KH, 4-5 drops GH), otherwise the water is way too soft for the vast majority of fish.
We drink the same water and use it for everything in the house - 110 ppm TDS, 1-2 drops KH, same GH (of course not out of the sumps but split off before).