I think a simple brittle star would do fine in a 25 gl. If you feed it regularly. They are generally part of the cleaner crew but i love the 4 i have. but they are in a 90 gal. So 1 might be ok.
i don't reccomend the green brittle star. I have one but he has gotten quite large and I have to make sure he stays full so as not to eat my fish! But serpents, grays, blacks and reds seem to do fine. They hide most of the day, and come out at night. but mine come out when they smell the food now.