One of the dwarf angels or the dwarf lions would look nice. Be careful with the lion, if it gets too big, it'll try eating the damsels and chromis for dinner. I also like my Blue Hawaiian Spotted Puffer. Real personable guy. He's picked on a couple of my snails, but other than that, he's been fun to watch. First to greet me at the front of the tank when I come by. If you don't get too attached to your fish, you can do what I did and get a small blue hippo tang (1" - 2"). It ends up growing too big for the 55, but once it does, you can use it for trade bait at your LFS. You can also go with the Purple Tang. They grow to a maximum size of 6". Get you a small one, and he can stay in there for at least a couple of years or more.