not sure of the difference between brittle stars and serpant stars.
I had what i think is a green brittle star (he didn't have smooth legs, they had bristles/hairs/feelers) that approached 20" and started to eat most new additions I would add to the tank.
he ate two clowns, two firefish, flame hawk - all new additons that didn't make it overnight - before I figured out what was happening
\fed the tank well