To give any recommendations as to proper clam care, we would need to know the size of them. As they grow, they need less phytoplankton and more light for proper nutrition/growth. If they are only 2-3 inches the most efficient method of feeding I've heard is to take them out of the tank, place them in a separate container and add DTs, Phytofeast, etc. In this way, you can target feed your clam and get a high concentration of food to it. Hope this helps getting you started!
Eric
PS- some clams do better on substrate, some do better on rocks. The Crocea prefers a hard substrate to foot on, while the Derasa may be kept on the sand.