The only one that might work for you is a brittle star, but those aren't the prettiest things...plus in a small tank, it'll go after everything (cleaning crew and fish).
There are a lot of other stars out there (linkia, fromia, sand sifter, etc) but no one really knows the diet of these stars and, as a result, very few have long term success with these animals.
__________________
32"x32"x20" Cube-ish tank
|