i have too either use springwater or distilled cause my tap stinks!!!!!!!!!!!!!
ill ask our LFS this weekend when i come over Smartdog...here are some stuff that i found about springwater
Springwater---You can use bottled spring water. Distilled water has too much of the minerals and stuff removed for long term use.
Distilled water---I do not recommend using distilled water EVER. Yes, distilled water is “pure” in the sense that it does not have any harmful chemicals in it. Matter of fact it has NOTHING in it. And that is the problem. See, bettas need some essential minerals which they get from their water. And distilled water provides none of that. It is also way too soft and acidic and not suitable. It is also expensive. If you are currently using distilled water for your jarred betta, please switch him to something else
Springwater------Bottled water, if the PH is within the betta range (see below) does not need to be treated with much of anything, since it has no chlorine. It also contains minerals, which is good. Store it in the room housing your betta so its temperature is the same as the betta’s jar. It makes it safer when you change his water. If you only have one betta, bottled water might be a good way to go.
Distilled water----NO! Betta fish, like all other fish, use their gills to absorb oxygen out of the water that they live in. In doing so, however, they also absorb certain trace minerals and other nutrients that are in the water, much like we do when we drink it. Distilled water contains no trace minerals or nutrients; it is completely sterile, and therefore has a lesser capability of harboring oxygen molecules (the molecules have nothing to "cling" to), not to mention trace minerals and such. Humans should not drink distilled water because of its' utter lack of mutrients and electrolytes, so think of it this way: if you can't drink it, your fish probably can't live in it. He'll almost certianly die if you make him live in distilled water, especially if it's free-standing water and doesn't have a filter/water pump.
http://wiki.answers.com/Q/Can_spring_water_be_used_in_a_beta_fish's_tank