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A recent answer suggested that if your home water supply was not used for filling a fish tank, that 'spring water (not purified water)' was preferred over distilled water.
What advantages would one offer over the other? What should I consider when choosing bottled water for my fish tank?
Distilled or reverse osmosis water does not have the minerals necessary to buffer pH. If used exclusively you will have a very unstable pH which is dangerous for fish.
Spring water can vary in composition, in terms of minerals and pH, again creating an unstable environment. Bottled water can even be chlorinated. Combine this with the instability and you're better off using dechlorinated tap water in most cases.
If you want to go down that route distilled/RO is preferred, but this must be remineralised.
References
Following page lists pH of various bottled waters, ranging from 3.37 to 11, highlighting the varying composition. http://phconnection.com/Bottled_Water_pH_List.html
Following article from fishkeeping.co.uk, a popular online community, talks about the use of RO water, remineralising and buffers. http://www.fishkeeping.co.uk/articles_109/ro-freshwater.htm
Article from a manufacturer of RO systems stating 'RO water has little or no buffering capacity. That means that the addition of even a small amount of acid will have a large effect on pH.' http://www.puretap.com/ph.htm
Article on water chemistry for fish keepers which states 'changing the pH by more than .3 units per day is known to stress fish.' https://www.cs.duke.edu/~narten/faq/chemistry.html
I'm a marine biologist and have inadvertently done this experiment with toxicology tests on fish. Distilled water alone in a very clean container will kill fish quickly. It's not the lack of minerals directly. It's the fact that salts in the fish's blood will rapidly diffuse out into the distilled water. This leads to neurological problems.
If you use distilled water I'd make sure to add some solutes before dumping a fish in.
I suspect for water changes it's not as big a deal. I'm lucky in that tap water where I live is fairly high quality so I wouldn't consider DI or RO water.
Reverse Osmosis with 0 TDS (using a DI filter) is the best choice. Bottled water generally has a pretty high TDS number from my experience (low 100s). High TDS in the water you choose will provide food for algae growth, especially once you throw fish poo into the mix.
As for PH buffering, there are additives for that if that becomes a problem. You did not state if you have a salt or fresh water tank. For saltwater, you just need alkalinity & calcium, which a solution can be made from baking soda (http://reefkeeping.com/issues/2004-05/rhf/).
If you do not have RO at home, most fish stores sell it for around $.50/gallon.
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 ...
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" ...
http://www.safewater.org/PDFS/resourcesknowthefacts/TDS_AND%20_pH.pdf
In short:
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.
