Pincushion starfish diet?

[Frostyeel]

My sister just got a largish one of these because they had a couple at our local fish store. I'm guessing it was a bad idea, especially getting one too large to fit in the 20 gallon tank, but I want to know if it is going to die and I should get rid of it asap or if there is something that we could feed it?

Google and RC searches have only brought up pincushion urchins mostly, or vague references to pincushion sea stars. I havne't been able to find any info on these guys.