yellow tangs can be aggressive, i would recomend putting the long-nose in first, if their is aggression it is because the fish are of similar size and the tang sees the butterfly as another yellow tang. Anyway i would add the LN Butterfly first and make sure it is much bigger than the yellow tang, hopefully this would setour any aggression, so that niether fish recognizes the other as a threat (more likely the tang will be the aggressor)