I had the same problem a few weeks ago. I had a clam in my tank that was not responding or closing when I would lift it. But it did not look dead so I left it alone and decided not to pull it out. The next day before I left to work at 5:30am I looked in my tank and the clam looked the same. During the day I looked up some information about clams on the internet. In one of the articles I read it said to pull the clam out and smell it. Another article said that the muscle of the clam is the first to go just before it dies. So the clam will no longer be able to close. I left work early that day 12pm when I got home my tank was so cloudy I could not see anything. All my corals were closed and a few fish floating and the rest of my fish were dieing. I pulled out the clam and it smelled SO BAD. I quickly transferred everything to another tank (took several hours). I lost a few fish and corals. But if I would have left work at my regular time everything would have died. I WISH I WOULD HAVE PULLED IT OUT!!!! Hope this helps