weeeeeellllll. i assume you are talking about those of the tridanca species and not ofther bivalves such as a cleaner clam.....
soooo to be able to care for them all you will need a mature tank of at least 8-10 months, but preferably over a year and MH lighting or a lot of t5s. you will also need to be ontop of your water levels esspecialy Ca and ALK. If the clam is under 3 inches, you will need to feed your tank, or bowlfeed the clam live phytoplankton at the least every 3 days or so. but preferably every day.