Looks like the convict was big to me. This is one of those things, that is possible, but probably not the most common occurance. I have also kept a footlong Oscar with a convict, and various other 5"ish cichlids, with no problem. But certainly possible for a fully grown O to try and eat a fish of that size. O's have expandable mouths, moreso than something like a melanurum. Whether you want to chance it, is up to you.