And then they were gone.

[npsossi@msn.com]

I have been working on a friends tank for the past month. It is 36 inches long and I am guessing a foot wide. All his fish had died before I started working on it. Last week I put a plug with 6 watermelon Zoas on it in this tank. The next day he called me to say that 3 of the polyps were gone and today the other 3 were gone. I am guessing he has a crab eating them. But he just told me he has a lobster in this tank. Could the lobster be the problem?
Thanks