You could get an Oscar or a Jack Dempsey. An oscar gets a few inches bigger then a Jack Dempsey ussualy but the average for an Osacr is 12+ inches so you could not keep them in there for life but you could for a while. Also since you are looking for an aggresive fish you should go with a JD over an Oscar if you choose one of them. An Oscar is not as aggresive as a Jack Dempsey it is just seen that way by some since it eats almost anything that will fit in its mouth and is always hungry. Good Luck though.