Clams can sometimes tell who is friend or foe. Watch your fish and clam, if your clam stays open when fish swim by your fine, if it closes when one swims by, this one should be the culprit. I had a Imperator that grew up in the same tank as a clam and where fine for over a year and then one day I noticed everytime the Imp swam by the clam would close. The purple tang could run into it and the clam would not flinch, let the Imp swim by and close, I moved the clam.