I always enjoy watching the scene when a school of Midnight Parrotfish swim by. The school swoops down on a section of the reef, attacks the hell out a spot for a bit, then suddenly swim away at great speed, like they have a schedule to keep.
There does not seem to be an overabundance of online information resources about the Midnight Parrotfish. The best printed source I have found is in the book Reef Fish Behavior by Ned Deloach and Paul Humann.
Deloach and Humann tell us that Midnight Parrotfish “feed on filamentous Algae scraped from the reef base”. Although there are many natural patches on the reef, algae is also cultivated by Damselfish. What happens is a school of Midnight Parrotfish, “overwhelm the defenses of the damselfishes who have no trouble protecting their personal algae gardens from solitary herbivors, but have little chance fending off a swarm of feeding fishes.” The books goes on to tell us that the have seen “distinct white bite marks on several egg patches on Sergent Majors egg patches. I would conclude that they eat eggs in addition to algae. I suspect that they overwhelm the Sergent Majors defending their eggs just like they do to damselfish defending their “farm”. This might explain one reason (I’m sure there are several) that Midnight Parrotfish school up and feed as shown in the video above.