From what I understand the betta water or other bottled/distilled water is not good for bettas since it doesn't have the needed minerals.
I know that Prime will take care of your ammonia for 24 to 48 hours, but after that it converts back.
Once your tank is cycled, it will take care of the ammonia on its own...Maybe you could use the bottled water jugs they sell at the grocery store until you get your cycle established...
I'm sorry I'm not more help, I've never dealt with this problem before with cycling. :( Halyx is awesome with cycling information, maybe they can help if you PM them?