I just held a measuring tape up to my tank, and the Oscar is at least 11" and almost pushing 12". The thing is my Oscar was totally fine with the Blood Parrot that I had in there (they even grew up together) until a few months ago. From what I have read, and gotten from others is that the adult Oscar temperment changes and often they decide that they want the tank to themselves. With this in mind I added the JD which turned out to be too aggressive for my Oscar, and I just couldn't let my Oscar get beat up. So for now he is my solo spoiled brat. 
