Is distilled or spring water better for fish tanks?
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.
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