Oscars get way bigger than twelve inches, I still think the combo of an oscar and JD is okay just filter the pejesus out of the tank. I have Oscar JD Blackbelt in a 90 with no problems (same footprint as a 75 a little more swimming room though). Other ideas are texas (carpintis) or motaguense maybe, might kill the JD though. Salvini and green terror were also good suggestions. How about geophagus brasiliensis or a red head earth eater?