Brittle stars are in most cases rather easy detrivores, though some of them can also be predators.
Unfortunately the reef safe sea stars (starfish is a incorrect common name since they are no fish) are very delicate and by no means easy to keep. Most are very specialized in eating bacteria films, so a mature and large enough tank is required.
Another issue with them is wrong handling anywhere on the way between the ocean and your tank. The thing they really handle badly is being out of the water and exposed to air.