I'm quite the newbie, so other folks will likely say more.
I would say calcium, to be on the safe side, and be prepared to add some (through one of the numerous methods) if necessary.
And, since you're modifying a fish only tank I would say slowly raise your specific gravity until you reach 1.024 or 1.025.
This is based on the (lots) of reading I've done over the past few months, talking to a lot of people, and my own (new!) success at keeping a small reef tank...so I've probably left something out. I'm sure that wiser folks that me can give more specifics.