Sand sifting stars have a poor survival record in small tanks. They tend to clean that sand too fast and too effiicent, and due to the small surface alrea/volumn, the biodiversity of the sand bed doesn't have enough time to recover.
A seastar is nice, but do pick one that will readily take food, i.e. the brittle stars as recommanded. Most stars are not suitable be it a 20G tank or a 800G tank.