RO/
DI water is better, but
RO is probably good enough.
FWIW, when I got my unit, they had gotten a bad box of
DI carts and the
DI was not doing anything, while waiting on the new cart to arrive, I used the straight
RO water with no ill effects, course that wasn't a suprise, I had been using straight
RO water for all my water needs for 18 months before I bought the unit