Do the small clams need to be fed?
Yes. Taken from LiveAquaria:
"In the home aquarium, the Maxima Clams require intense lighting to thrive as they contain the symbiotic algae called zooxanthellae, and receive the majority of their nutrition from the light through photosynthesis. Smaller T. maxima that are 2” or less in size are much more sensitive to very intense lighting as their membrane is much thinner than larger specimens. For this reason care should be taken to properly photo-adapt them to the existing reef aquarium lighting in a similar manner to newly introduced stony and soft corals. When adapting a new Maxima Clam to very intense lighting, it is ideal to set them on a small rock or in a plastic dish with coarse substrate at the bottom of the aquarium. Over time the clam can then be slowly moved up higher in the aquarium.
Tridacna clams are also filter feeders and constantly filter the water for small particulates. Maxima Clams larger than 2” do not require supplemental feedings, but smaller T. maxima that are less than 2” should be fed a phytoplankton or greenwater supplement several times per week if maintained in a nutrient poor reef aquarium."