after reading this, I went to my local grocery store. They had clams for 69cents each. They're huge, about 2.5-3 inches each. I acclimated them for about 20 minutes (floated). Then dropped them in. One in my sump, other in the DT. The one in the sump seems to be doing well. The one in the DT started burying itself in the sand, but only got about half way. Today, I looked in there and a large chunk of it's shell is missing. I think my zebra eel took a crack at it.