It depends on the size of the starfish, size of your tank and how long your tank has been established. I've got about 5 baby ones hiding in the live rock. They came with the rock, which was from an established tank and are roughly the size of a 10p coin. Most starfish die after a few weeks or months in new tanks, as there is insufficient food for them to eat and target feeding can be problematic, as other CUC like hermits get to food before the starfish do. I personally wouldn't keep another starfish in anything smaller than a 4-5ft tank that had been established for over a year.