There is a lot more to keeping clams than just light. Ca, Alk have to be established, and stabilized as well as pH, salinity, and temperature, and in nano tanks this is not a easily accomplished task.
Almost everyone who understands these animals will tell you that short of metal halides, there is not much you can do with clams. The problem is that flourescent light just does not penetrate the water as well as a metal halide.
IMO, clams in a nano tank are best avoided or left to very experienced hobbiests because the clam need such stable conditions, and the clam that is to be placed into a nano system is going to be very small by clam standards. Again adding to the difficulty level. Small clams have a poor survivability rate in captive aquaria as they rely HEAVILY on phytoplankton as a food source
If you want to make the Clam a pivotal species in your marine tank, than design the system around the clam from the ground up. Get a tank that is big enough to house it when it grows, because given the proper conditions, clams grow rather large, rather quickly. Invest in the proper lighting to create as close as possible a natural environment.
I have done some rather extensive research on clams, let me know if you have any questions.
HTH
Justin