Came home from work today and noticed somthing in my tank.got the 29g running a short cycle with my live rock in it and noticed somthing sittin at the bottom of the tank.I pulled it out and took a look and it looks like a clam,but were did it come from? there seems to be nothing in the shell,its empty and half open,its in the tank now after like a 3 sec look at it,do i keep it or throw it out?and were did it come from??