I'd get a pair of angels with a lot of live plants in the middle. Since you have a taller tank, angels would be great in there. It may look pretty empty at first, but if you leave them room so they can grow to full size, it will look really nice. You could put a very good-sized shoal of otocinclus in there as well. You may want to consider some live driftwood for these guys, especially if the tank is newer. They do not do too great in unestablished tanks. If you don't want angels, go with discus. I don't know enough about these to recommend them though. But angels would be great. =)