I agree completely with acrodave. Generally, you don't need to add anything extra to a tank specifically for the clam. If you do decide to add phytoplankton (like DT's phytoplankton), just make sure you broadcast feed it to the entire tank. Don't direct it specifically at the clam as that would be far too much phytoplankton for the clam to handle all at once. As acrodave said, lighting is the real key (OK, he didn't say that specifically). As long as you take care of that and water quality, everything else should take care of itself.