The first photo shows him expelling his zooxanthellae - which happened AFTER you got him in your tank. He looked pretty healthy to me in the first couple of photos. I would worry more about your tank environment before you tried another sand anemone. These critters are used to being buried up to their oral disks in sand or coral rubble. Their column/foot is very soft and defenseless. I don't doubt that your fish were eating it - and there was nothing it could do about it