It is easier to make friends with like minded creatures.
Herbivorous fish, like parrotfish and doctorfish eat algae and are programmed to stay away from creatures so large as to swallow them whole. Even large parrotfish, who may not be afraid of us are aloof as their food, algae, is everywhere and they do not see us as having anything of benefit for them. Some fish have specialized niches so we are not on their radar. We see some of these fish on a regular basis and others, who do not live on our reef, as a rare treat.