I love the black. When you keep it all clean and nice it gives it sort of a "captive feel". It frames all the things inside, and for me I feel like it makes it look more like a work of art rather than just a piece of the reef dropped in. I guess its hard to explain what Im trying to say, but I like the black.
What are your plans for the sailfin when he gets a little larger?
My mom has a 125 tank that he could be re-homed in. I'd never keep a fish that was too big for the small space, but given the diagonal space in this tank, even though it's only 2'x2' he seems very happy with the space for his size for right now. If I see even a hint of lateral line or scale loss, he'll be moving to a larger environment...so far he's fat and healthy.