It certainly is a starfish! His name is Patrick, he's a Batik starfish.

I don't know the precise dimensions of the tank, and I probably should,
so I appologize. My lights are 36" across, so the tank is roughly 3ft across? And the depth I'm going to say is about 18", and the height is about 24"? I measured with a ruler, it's the best I can do.