The clam is still looking great in the QT, so posting a pic of it now won't help with a diagnosis. You'd just see a healthy clam. I don't have pics of when it or the others were hurting. However, when it was struggling, the shell was not closing up like clams do when a fish pecks at them. It was still open, the mantle wasn't extending up to or beyond the edge of the shell. Like I mentioned, you could see the inside of the shell above the mantle's edge. I've seen clams that are upset by fish, and they usually close up their shell and pull their mantle into it. This was not how my clams looked, too bad I don't have pics of it
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Also, I have lots of rock in my display tank and very little room on the sand. As a result, there's no room for my clam to be on the sand with some sort of transparent barrier around it to block out fish. This clam is too big to fit a 2 litre bottle around, so I'd have to find something bigger than that.
I'm hearing from local people that several people are having this problem. Unexplained clam deaths in healthy, stable systems.