RO/DI Ok for Betta ?

[Myka]

It will take a while to kill him anyway.

At the very least, you could do him a favor and use Aragonite sand if you're going to stick with the RO/DI so there is at least some amount of pH buffering capacity.

Fyi, in most cases you can use the waste line of the RO/DI unit for most freshwater fish.