Clam death.

[Skim]

Clams are funny they don't give you much of sign saying I'm in trouble. If they are being attacked from the bottom you will usually see then open and closing quite often and kind of twitching from side to side and they will also stay closed when you would expect them to be basking in the light. They have to be treated almost like softies they like there Ammonia and Nitrates and probably Phosphates to boot. If you ask some that have a large clam like 12 inches or more or have many clams they probably have the cleanest water as in Phosphates and Nitrates. I met a guy in the LFS one day and he had a 280 Gal I believe and had nothing but Clams in it the whole tank everywhere, he showed me photos it was amazing but he did not skim did not use RO water and water changes where far and few and he still had to dose Ammonium and Nitrates to keep them alive and the more he dosed he said the more colourful they became and grew much faster. I know from the past if you have Bristle worms your Clams will usually die, they drill there way in from the bottom and eat the Clam from the inside. What a way to go.
Anyhow sorry to hear of your loss. Like I said I have lost some beauties in the past all to Bristle Worms once they get a taste the rest are doomed.

All the best.

Mike