Why are all of my clams dying???

[doch]

OK... seriously frustrated.... wait... no, MAD!!! I had a Squamosa die a couple of weeks ago, then I come home from work this wek to find a dead Maxima, then go out today, come back and I have a dead Crocea. WTF??? All of my parameters are good... (Phosphates - 0.02 Nitrates - less than 5 Calc 400 Alk 8 Mag 1250, no amonia, no nitrates) On the 2 squamosas in the sand (one still living) I found bristle worms, but no pyramid snails. Can't tell about the Max and Croc, because up until they dies they were stuck solid onto the rocks. I do have a Niger Trigger, and a Flame angel, but I'v never noticed them nipping at all. Any thoughts or tips? Before I came home today I bought 2 more clams... I don't want to lose them.... please help!! Thanks!

BTW I've heard that a lot of people in Edmonton are losing clams lately... not too sure on the validity of this, but there it is.