I got those two new clams a few weeks ago. One of their mantles has been slowly bleaching (it's on the bottom, on the sand bed) and now is retracting it's mantle all the time. All my other clams are doing fine, and have always done fine (I've had some of them for a really long time). I've tried positioning it in different light intensities, with nothing helping. Should I move it up even higher, onto the rocks? Dunno, maybe I just got a dud, a lemon, a flop!!