Bettas and R/O water

Question
Just listened to a talk on raising Bettas and these experts said to use about 75% store bought distilled water and 25% tap water. They specifically said "Do not use R/O water.  Why would this be?  We just spent $200 for an R/O unit for our salt water tank and had planned on using it for a 125 gallon Betta tank. Store bought distilled water, doing 20% water changes, could get costly fast. Is there some reason that R/O water can't be used in a Betta Tank.
We are just checking our options before starting this new adventure.
Thank you.

Answer
I do not know that the reason is exactly but all that work on water is pointless.
Regular tap water with water conditoner added works just as good. Water Conditioner can get costly though with that big of a tank. Why do you need a betta tank that big? Are you planning to breed them? If you aren't, than you don't need that big of a tank. A 3 gallon works good for a Betta (can not have bettas in the same tank).
One thing that is a common misconception is that bettas but be by themselves. This is a lie, Bettas can be added to any community tank (no sissortails though, they WILL pick off the fins).
R/O can have bacteria build up in it and that will eventually leak into the tank if the unit is not cleaned regualarily enough.