TC, a 75G would be fine for a single Oscar and Severum. IMO a 125,180, or 220 is just silly. Good god I saw some of the people on here tell a guy a few days ago that he needed a 55g for a single JD. Shakes head.
Seriously I have an Oscar, jd, severum, in a 75g and they are healthy and happy as well... People on here think everyone can go out and get a 2000 gallon tank to hold a single fish. Do ur water changes and enjoy your fish, this website can sometimes be a little to high and mighty