Assuming you're looking at the decorative clams, squamosa, maxima, etc. Depends on the species. Most need a minimum of about 5 wpg, and up from there. Clams get almost all of their food from the zooxanthellae, meaning they do much better the more light is available to feed them. I do know of a few folks who have sustained clam on minimal lighting by placing the clam very close to the surface--but, IMO, this means you can't really see the clam from a good angle--you miss seeing the mantle!