the other fish leave it alone for the most part, and he has a bit of an attitude when anything comes near his territory. i'll bet your puffer would have a hard time bothering one, they are so quick and agile and swim in all different directions; upside-down, sideways, front, backwards, and they change direction on a dime. really one of the coolest fish to watch swim and maneuver around.
the only other thing i've been able to come up with is astronotus crassipinnis, but there's not a whole lot of info out there on them. so for now, oscar it is.