clam killers

[xdusty6920]

well im kinda stumped here. i got a small derasa about 3 inches from another reefer who had it for a while. it was completely healthy but after just a few days he died. i know what your thinking and no it wasnt params or the acclimation. i use a 3 hour drip acclimation and have zeros across the board with a constant 80 degree temp, 420 ca and 10kh. its a mature system with 250watters lighting it.
anyways after the small derasa died i didnt think much of it because it was new and so small. a few days later my derasa (had for almost 2 years)thats about 6 inches started closing up alot and acting funny. within a month or so he was also dead. i chalked it up to irony and told myself there was no link between the two. and now finally this morning my big squammy thats been with me a while is now dead too.
the only clam still in the tank is the oldest clam (crocea). the one thing that different about this clam is its at the top of the rock work and the others were all on the sand bed. maybe this has somthing to do with the answer? my best guess is i have somthing that preys on clams in my tank but havent the slightest clue what it could be. i really love clams and this is really bugging me. i wont put another one in that tank until ive found the problem. please somone help me figure out what killed all of these guys.
btw it wasnt minched mantle. all clams were perfectly healthy and then suddenly stoped opening and then died shortly after.

 

[Kalkbreath]

What salt do you use?
What buffer?

 

[mbbuna]

list all the fish,shrimp and crabs you have

 

[nickyblase]

bristle worms maybe?

 

[xdusty6920]

i use IO salt. and use kents superbuffer dkh (but dont really need to use it much). there are no fish in the system, some blueleg and redleg, and 2 comensial shrimp that live in clams (clear with dots kind). upon finding each one dead they were over ridin with bristle worms. now i dont know if they were the problem or if they just were munching on the freshly dead clams. ive checked them all several times for snails and that was never the case so i cant figure out what was killing them off.

 

[mbbuna]

bristle worms wont kill clams.

how were the clam in the weeks leading up to there deaths?were they fine and then one day dead? or did you notice any changes?

 

[xdusty6920]

the only one that started to show signs of distress was the 6 inch derasa (who was the second one to die). he just started only exposing a half inch of mantle all the time and then started closing all the way.
as for the other two.......no signs at all. big, open, happy, extended as always........next morning............dead.
could it be a predator? could it be disease?