A full grown male jack dempsey can top out at 8 inches. Oscars can grow 12-13 inches. Keeping them in a small tank that is not suitable for their size not only will lead to territory issues, but can also stunt said large fish. I'm sorry if you feel inclined to disagree, but as someone who has owned oscars, and breeds jack dempseys, I'm afraid that you are going to run into problems.