clams in general need a lot of light, however some are a little less demanding than others. typically the clams of the species t. derasa, and t squamosa, are a little easier to keep while t. maxima, and t. crocea are much more demanding (these are also the most beautiful af the 4) Light is so important because it fuels the photosynthetic zooxanthellae (tiny algae that inhabit the mantle of the clam), and these algae then produce energy which the clam uses to grow. clams also need a great deal of calcium to grow. what are the specs on your tank?
good luck
jon