[Reefie]

I may not be as knowledgeable as some of the Canreefers on here, but I have a bit of experience with clams.

Clams close up if there is something that is bothering them. I am going to guess it is fish related, the strawberry basket cage is a good suggestion.

Having read your post about 0 Nitrates, that's a great thing for SPS, but not for clams. Clams consume Nitrates, so having a little is beneficial to them. I've had up to 11 clams at one point, yes I got hooked on them. I am down to 5 now, there is a delicate balance to keep clams and SPS.

SPS like "CLEAN" water, while clams like "DIRTY" water. I have found that to keep both SPS and clams, you have to keep the water parameters almost perfect for the SPS while dosing Phyto and or Sponge Power to feed the clams. I've got 2 Crocea's that I've had for about a year now, got them when they were about 3" and now they're about 5"

Another thing could also be your CUC, I lost a few of my clams due to them picking at some algae that was growing on the shells. It iritates the clams when hermits or even your cleaner wrasse is picking at it's shell.

I would suggest that you do 10% water changes weekly and see if it makes a difference also. Clams like everything else we keep in our tanks prefer stability, so make small changes gradually.