I might be out to lunch on this as I'm still learning the water chemistry. But I was thinking that if the RO/DI removed most of the minerals and impurties from the water it would make it very soft as in dkh = 0. That might be fine for discus or something needing soft water; but, try having a mbuna tank and keep a ph at 8 with nothing to buffer the water?
Am I missing something?
At any rate. For saltwater, keeping to forum, just adding mix should be all that is needed.
Cheers