I introduced a tiny (about an inch long) cleaner wrasse into my tank yesterday and my existing fish (all bigger than it - minimum twice the size) run away from it when it tries to clean them. Very amusing to watch. So far I have seen the tomato clowns, purple firefish and the blue damsel all run. It hasn't tried to clean the sixline wrasse or the decorated goby yet (well, if it has I haven't seen it) Good thing the cleaner wrasse eats prepared food also