Originally Posted by VJM
I know bettas need the minerals in tap water. I don't know if bottled water needs to be remineralized. Definitely something to look into.
Depends on the type of bottled water used. As far as the various brands of bottled spring/filtered water, quality can vary a lot, often its still necessary to use a water conditioner to remove chorine/chloramine/heavy metals/etc. that are actually still present and I'd definitely test the pH levels, hardness, and any other pertinent parameters prior to use as well. Distilled water should be avoided in general. Personally, if you're going the bottled route, I'd recommend RO water (filtered via reverse osmosis).
Like a couple of the previous posters, my water has quite a relatively high pH--8.0-8.3 I believe, but all my bettas have adjusted extremely well, to the point of producing spawns that grew into healthy adults. Keep in mind that high pH does NOT necessarily indicate hard water, while my water is very alkaline, its actually extremely soft at the same time.