The microfauna they feed off of in the water column is not there for them to eat in most tanks. They feed off plankton and detritus in the water column. You would need to feed live plankton like you feed some corals. It's possible pre-cultured green water would work as well.
I just read on Thekrib.com that they actually shouldn't be seen if they're alive, and that you will only usually see them when they die and their shell shifts to the surface after they die. The mortality rate is one third of all shipments of clams die prior to arrival and the rest will live at least a few months. They do require highly specialized setups, species only will most likely only work as their are a few precautions. Most fish will nip at anything sedentary and alot of bottom feeders would prove to be a problem to clams.
there's also this that I found.
"Another warning: Some clam species ALLEGEDLY produce zooplanktonic young who may become lodged in the slime layer of slow moving fish. The zoo-baby clam burrows into the skin and dies, but causes the fish to surround it with a white cyst. Now here's the rub; the cyst LOOKS like ICH"
