Its hard to make such sweeping statements about a group as large as the Ophiuroids (fancy name for brittle stars and serpent stars). Some are known for eating fish (Green Serpent Stars) some couldn't possibly eat a fish (Amphipholis squamata). But generally, a serpent star/brittle star CAN eat a fish, but wether it WILL is usually dependent on the environment, and the individual animal.
And your welcome, always happy to help.