I could be wrong (probably am) but from what I understand most all
FW fish will be fine in a SLIGHT BW.
I was under the impression that fish that are
FW/BW will do fine in either mixture, and anything in between.
My fish guy told me that he would do fine in the
FW, BUT... Any type of injury (brushing up against a tank decoration) that would scratch his slime coat and he would chance an infection.
Im still thinking about it. I need to do more research.