Hmmmm.. I will preface by stating i am not an expert. I have no concrete evidence neither, but ill bet there was something unknown on a clam that came in that has decimated your clams. I think this can happen to corals as well. I used to have a 180 gallon reef. It was pretty darn nice. I loved it. then I got AEFW. I took drastic measures and dipped the entire rocks (major encrustation) in Bayer. through that I killed my entire pod poulation but the tank recovered nicely. I got back on track and had some extremely nice growth. the photo below was about 6 months before something went completely wrong. several of these colonies added 3 or more inches beyond this.
Anyways, 4 months after this photo was taken, it all went downhill. all of my sps started to STN and I could not stop it. there were no bite marks, no signs of bugs, Nothing. I was so heartbroken I tore this tank apart and started over in a 66 gallon tank just using the two massive rocks on the left of this photo. For the first 2 years of this tank, I killed every single SPS i added to the tank. Nothing would live if it was a stony coral. LPS, Softies, Zoas, Anemones and a Derasa did fantastic. Montipora, acropora, stylopora, millipora, NOTHING like that would survive.
only in the last 18 months have I been successful with them. I am finally starting to get growth from slimers and stag type corals. Millipora and regular Acropora still lag behind. It has been quite a challenge let me tell you.
I wish I could help with the clam issue you have. I lost my derasa a few years ago due to a lighting change where it did not agree with my Radion (neither did the two aquacultured maxima I got from Pacific East Aqua). I now have a pretty black maxima under my Ai Hydras that seems to be quite locked in and happy. I need to check its growth but it has looked great for the few months I have had it.
