I've never seen a white background, but googled it and they do make everything look super clean. Definitely something to think about!
The mirror idea is cool too, never even crossed my mind.
I've looked into the shadow boxes and they are really cool, you're right about the plumbing though, might be an issue. I'm thinking too, what if we do an opaque/textured tint and hang some LED's behind the tank for a back lighting effect. Or maybe even use rope lighting around the tank border in the back. Not as cool as a shadow box, but I do really like the pics I've seen with the light coming through the back of the tank.
Thanks everyone for your suggestions!