Size doesn't matter, a jar is a jar. Put a big jar in a little jars tank and the small one will attack a bigger one without thinking, until the bigger one feels like it's his home and fights back, but by then all his fins will probly be shreaded, it's their territory not the fish, but sometimes it's a bit of both, they can be racist ha. Mine recently killed a 30cm jag because I didn't introduce him into the tank properly, relentless attacking, I added some more fish took my driftwood out and the tables have turned, he's fine again, if anything my clown knife chases him now, another territorial fish, but the tank is relatively peaceful now, jars are one tuff awesome Aussie predator, u jus gotta try contain their aggression and their fine