Many people believe that going out and buying distilled water, or Reverse Osmosis, will be good for betta fish. Think about the natural environment. (This is what you should always be thinking about when trying to create a home for your creatures). You don’t often see fish swimming around in distilled water. Often the water is something you would never swim in yourself, much less drink.
The problems with Reverse Osmosis water or distilled water is that it is so clean that many of the minerals found in tap water are gone.
With tap water you’ll want to treat for chorine/chloramines, whichever your water utility uses.
The best way to tell if you water is fish-friendly is to test for PH. In most cases, A steady pH is much more important than a “target” pH. A fluctuating pH can be very detrimental, whereas a fish can get used to a “higher” or “lower” pH with some time. A PH level of 7 or 8 is about the max you would want to see.
Chances are that the betta fish you buy are from your immediate area….from a fish store is likely to have the same (or similar) water source and have been acclimated to the water already. However, is is very important to follow the acclimation process below.
It is good information to read species profiles on the betta you are going to have to identify target PH values. However, remember these are for fish living in the wild. You will usually be purchasing fish who have been bound and captively bred.
Specific pH comes into play when trying to get certain species to breed (and even then, it can be questionable).
One thing that can be said on this topic is that it is not pH problems that kill your bettas.