Starfish are sensitive to poor water quality. I was told to wait until the tank has been established for 4-6 months to ensure minimal disruptions in water quality after the starfish is introduced.
i'd watch out for starfish booya those lil dudes can mess your tank up when they kick the bucket, i had a orange nob starfish for a month and in the 2 days it died he released a ton of ammonia for somthing smaller than a silverdollar