I recommend dwarf cichlids such as Convicts, Sajica, Cutteri, Salvini or Rams. If you want a mean little son of a gun go with Neets. I feel fish like Green Terrors, Jack Dempsey's and any fish that can reach 10+ inches should be housed in an 18" wide tank so they can comfortably turn around.
By going with Convicts, Salvini or Cutteri you could actually keep 2 of these species in your tank plus some dithers like a school of tetras or barbs and have a much livelier tank than just a single Green Terror.
By going with Convicts, Salvini or Cutteri you could actually keep 2 of these species in your tank plus some dithers like a school of tetras or barbs and have a much livelier tank than just a single Green Terror.