That would depend on a few factors
.
How big is your tank?
How much water flow?
How long has your tank been stable (they are fairly sensative)?
What fish are in the tank?
What type of Clam (have you researched the specific types)?
As you stated, they are filter feeders. In most cases you would need to feed the clams food stuffs in their partical range. This might be phytoplankton (green water).
Chris