I've done freshwater in the past and your reasoning seems sound. I travel all over the Southeast for work and I've never seen a freshwater clam in any of the stores I've visited. Admittedly, I'm paying closer attention to the saltwater side of things but I do browse freshwater. In the last 20 years the freshwater hobby has grown to include so many different species including a variety of shrimps and crayfish. It does make me wonder where the clams might be. My only thought it that there aren't any "outstanding" varieties that can be kept in captivity. I've heard of clams cleaning a crazy amount of water, maybe a relatively clean 55g freshwater setup can't support even a single one.