sveral things could have happend, when was your last water change, what did you spot feed and how many times did you do phytoplankton live or bottles or have you not, dependent on what kind of clam you had weather it was squamosa, dersa, crocea, or maxima some preffer bare rock such as maximas and some preffer sand to bury their foot and some like both like crocea. also some dont need as high as light squamosa and dersa dont need the same light intensity as maxima or crocea. hope some of this helps