As long as you don’t get a red or green brittle star, they are harmless. Brittle stars will pick a crevice and stick their legs out to catch floating food. Serpent will move about the tank trying to grab things. They are really cool to watch move and eat. Definitely a cool thing to have. I will advise you don’t get a serpent star if you have any larger crabs (like emeralds). They pick at their legs. My emerald killed my serpent.