I had a banded serpent starfish take a ride home with me in a rock that I purchased. I know that they can grow to be 8+ inches, averaging around 12". Is a 65 gallon tank too small to keep it in?
Should be fine. I had two in a 90 for over ten years and they were fine. Great scavengers that will get all the uneaten foods, but if it's out during the day or its arms are out in the current from the rocks it's hungry. Even though it's not as big or as aggressive as green brittles it will eat small sleeping fish to survive. I target fed mine at least once/twice a week.
I have one in a 65 for 6 months now, probably 6-7". I rarely see him, so sometimes I get worried...but he always reassures me about once a week, mostly during nighttime feedings. Have fun, cool to watch