In a tank that is tall like that you are going to want something to fill the upper levels of the tank. If you want to do a biotype tank, look at Festivums or a large group of 12-20 shoaling tetras of your choice/. If you are not looking into a biotype tank. I agree a nice shoal of 12-20 Congo tetras or some kind of rainbowfish would look awesome. (Your wife would like the rainbows, most likely). You have so many options but remember to look at where the fish spend most of their time to balance the tank out.
Good Luck and Take Care