damian, get your oscar back.... he wont need a new tank for a year. I've got one and many others in a 29 gallon. Hes vibrant playful and is absolutely fine. If hes lying down or gasping for air then you may need some more disturbances at the surface of your tank, that way oxygen makes it into the water. You could lower the water level so when the filtered water hits the surface more agitation brings o2 into the water. I have 8 fish in my tank, 1 of which is a 7 inche oscar, like yours, hes fine and is awesome, so get him back until he grows too large... or kick out convict and give him his own apartment.