A Pygmy Cherub Angel or African Flameback Angel would be a nice addition to your tank but can get beligerent. There are also a couple of wrasses that would work for you such as the sixline mentioned above or something like a leopard wrasse. There are also a lot of basslets like the royal gramma that would work as well.
Keep an eye on the coral banded though if adding smaller fish...
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32"x32"x20" Cube-ish tank
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