its far more likely that a brittle star will eat small fish, i have never heard of a normal serpent star doing that.
as far as there diet, they are considered part of the clean up crew, they will scavenge for what they can find, that said they will also take just about any ting you give them. left over fish food, shrimp, clam, etc..
yes they are nocturnal, but will commonly com out during feeding times to catch some left overs..
