Bristlenose (Ancistrus species) pleco if you only want one fish. They are happy to live by themselves or in a small group. They only grow a few inches long , but are both tough and peacefull. Extra bonus is they will help keep algae under control.
The Corys or Otos like living in a group, but then they are small and you could put 4 of each in a tank that size and hardly notice them