slojmn1 - the reason that I believe it is something which stays in the tank is: My clam deaths occurred before I had a mjaor tank disaster in which the tank almost completely drained accidentally. I filled the tank about 6-8 hrs later with cold tap water mixed with salt to try to save something (I had an empty 500 gallon tank in front of me on a day that I needed to be at work immediately and did not have 500 gallons of RO/aerated water available) Anyway, a few fish survived, and a few corals (amazingly, an acro which was dry for 8 hrs and then received a jolt of 65 degree tap water actually survived) The clams had all been dead several months earlier. After getting my tank stabilized again 3 months later, I tried two clams and both died, one at a time, 4 days apart, about 2 weeks after first getting them.
danmhippo - I also once cracked open a dying clam (hard to do to a still alive $100 specimen isn't it) and also found a small crab - almost albino - I think that I may have a picture. I figured that it was a coincidence and that the crab was preying on something that was already dying. Did you say that you put a niger trigger in your tank. Wow. I have a hundreds of small hermit crabs, shrimp, and other little things in my tank which probably wouldn't take kindly to the trigger. Anyway, is it common knowledge that such crabs are a problem?