 There is a debate on whether they are beneficial or not. There are people that swear they eat coral tissue and other beneficial stuff and some say they eat algae and that's it. One thing is for sure, they reproduce fast! When you see one without an arm or two, congratulations... you have new stars on the way.
I prevent a plague by removing them everytime I see them on the glass (anywhere where they are easy to get). For everyone you get out, you might have a lot of stars on the way, so take them out when you see them.
Oh, I forgot to mention, they stay small and do not grow bigger than about a 1/4" or so.
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30g reef tank, CPR CY192 filter w/ DIY plumb running Rio 17HF return, Coralife 3x 9w UV Sterilizer, 3 24W T5-Helios 10K Daylights/3 24W T5-Helios Blue lights, 2 Logysis blue meteor light strobes (moonlights/24 Blue LEDs).
Tiger tail cuke, asst. snails/hermits, asst. feathers, rainbow acan,zoos,shrooms,bubble,galaxea, asst. shrimp, 2 ocellaris,mandarin,zebrasoma xanthurum,pink-spot watchman,red-striped pistol. | 
Are these good starfish? 
