Reefer's Dreams

Went to the LFS today on my day off to see what they got in their shipment, and I noticed they got a couple of anemone, which would be good for my clown to live in. So I picked one out seeing the size of it in their tank, being about the same size as a closed fist, thinking that was the approximate size it would be. Got it home, put it in my tank, and turned the lights on. About 3 hours later, the anemone has at least tripled in size, almost overtaking the tank. I am not sure wether or not it is going to get any bigger, but it is freakin huge. The LFS sold it as a long tentacled anemone, so if anyone knows what it actually is called, please tell me. Here are a couple of pictures of it.
anemone
tank with anemone in it