My Crocea, which has been in the tank over 2 years and has shown excellent growth, has stopped opening. It was always wide open in the tank about 4 inches from the water surface. It was not moved and does not appear to have been attacked by anything. All water parameters are the same as always, and lighting has not changed either. The clam is under 250W MH with supplemental actinic NO in a 75 gallon tank. I looked at the clam and it does not appear to have any damage or predation. It opens slightly with the lights out, exposing its innards, which all look OK. There are no signs of any attack. All supplement additives are the same (DTs, Ca and alk from SeaChem). Any suggestions? I would hate to lose such a great creature, which has grown and florished in the tank. Any input and suggestions would be very much appreciated..........