Clams are exceedingly sensitive. Honestly, I think if you have one >1 year in your tank, you're doing really well. There are so many things that can do them in without you even knowing it. And the problem is, the mantle can be responsive well after the rest of the clam has taken ill, that they can look to be healthy and doing well, long after the point they are "essentially" dead.
Another thing to watch out for, once one goes, it very frequently starts a cascade effect, and others in the tank alongside (even though may have been healthy to begin with) can be taken down in the aftermath.
Very frustrating, and disappointing, since they are so nice to look at.
Sorry to hear of your loss Walter.
