Clowns can be super agressive in small tanks, they attack anything that comes to close to their eggs or nems (or corals or w/e they host), I've seen clowns dash out of the anemones to attack a emp angel that was easily twice its size. That said, in larger sistems, something around 200gal is one im thinking of specifically, I have seen Marron clowns hosting a LT nem, and they were the nicest politest clowns I've ever seen, they only attacked the fish If they got REALLY close to the nem, but they are supposed to be the most agressive of the clowns. SO my belief about them is the same I hold for tangs and other "agressive" fish, is that they are only really agressive when confined into tight spaces with others, or when new tank mates are added. Hence my following the saying Bigger is better.