From my own experience I have found that just because you have been able to keep an aggressive fish with a smaller non aggressive fish does not mean that you will be able to put a new tank mate in and have them work out. This is especially true if the first pair were raised together. It is likely that the oscar does not realize that the convict is a nutritious source of protein, whereas if you put a new fish in, even bigger, your oscar will start thinking of garnishes and lemonsauce...
