The source is sort of irrelevant, the process should still be the same to ensure the safe well being of your tank life.
The full process should be ...
- Take water from almost any common source.
- Push through RO filter to get as close to pure water as you can
- Add salt
- Add air to water
- Align temperature with that of tank
- Add to tank (preferrably slowly to avoid shocking life in tank)
Step 2 is particularly important as it gets the water in a known default state allowing you to be absolutely sure exactly what you are putting in your tank.
To be clear here ... you should ALWAYS know exactly what you are about to add to your tank and never take a wild risky guess.
Would you go swimming in water that you weren't absolutely sure didn't contain harmful elements to you? ... no ... then please don't ask that of your tank inhabitants.
I say this because every day we see news stories about various companies making risky or just plain dumb decisions to provide ever cheaper food in our supermarkets, and water is one of those products so how far do you think a water company would go to ensure that water got to us for the cheapest possible price?
Legal guidelines on water are not about "quality" but more about "taste" (at least that's how it is in the UK).
When I was doing my research on tank water years ago setting up my first tank I found laws stating things like "TDS of 300 is excellent" ...
What works for / won't kill a human is most definitely not good for a fish or coral.
Please for the sake of those poor fish, do the right thing and always start with RO water.