Large brittle stars? I don't suggest them if you keep small peaceful fishes. The stars tend to eat them. They go into "hunting mode": they tent their bodies up, resting on their legs, and when a small, unsuspecting fish swims under them, they drop down on it and eat it.
Shimek's Pocket Expert Guide to Marine Invertebrates dubs them, "Green Death".
The smooth-legged serpent stars are a better choice, as they seem to be less likely to prey on your fish.
The tiny serpents and brittles that have an overall diameter the size of a quarter or less (legs included) are harmless and are actually good janitors too. These don't grow larger than that, and are beneficial.
Jenn