As mentioned, green brittle stars, and larger ones at that, will trap and eat fish. They set up in a trap like posture and wait for fish to swim below them. They then pounce on them and consume them. I've not heard of any problems with other brittle stars. I've never heard of this problem with serpents. The big greens tend to be the culprits. However, ANY predator will prey on your livestock if not properly fed. I have a serpent and brown brittle in my tank. I ensure that they are fat and happy by target feeding them. If they are fed properly, they will probably overlook your fish as a food source. If they are starving...anything is game.
David