I have a derasa clam which I thought was doing great. But over the last few days it remains closed for most of the day. Opens in the morning for 2-3 hours then shuts up. I can't see it being bothered or nipped at. Some details: Lights are pfo led lighting, clam is one third up - getting lots of light. Water quality good: no detectable nitrate, nitrite or ammonia. Everything else doing great. Should I be concerned ? Anything to be done ? Thanks for your help.