Mine eats shrimp alot. It used to fight my scorpion for them. They are primarilly herbivores, and tend to eat algaes, sponges and the like. There is always the occasion where they may decide that they like other foods! For as peaceful of a fish as they are, mine's a tyrant.

Have you checked you alk at night? You could be going through a ph/alk swing from lights off to lights on that could be causing it. It could be that you have one of the harmful bristleworms, or a rogue voracious starfish that is stressing them out after dark.