what fish

Hi all, I just added a pair of Percula clowns (post-QT) to my recently cycled reef tank. What would you add next, and why? Tank has a ~68 gallon display with lots of dry rock and a 2" sand substrate, if that influences your choice.

So I was wondering, what should I get next fish wise. I have a 13 gallon with a pair of clowns and a cleaner shrimp + a clean up crew. I want to get a Exquisite Firefish, as my centerpiece fish. But I also want something that is productive for the fish tank like a sand sifting goby. What do you...