I have been told by reef aquarium experts that clams do not require metal halide lighting to thrive. While MH
is optimal, a clam can live and maintain color and vitility under Compact Flo. Lighting, VHO
lighting, and other types of high output light. The most important thing is that you have the wattage and the spectrum. For most clams 4-5 watts per gallon, especially of non-MH
light is pushing it. I have two clams a Dersea and a Maxim under 7.5 watts per gallon of compact flo. lighting and I have seen impressive growth. My tank is a 75 but its a shallow tank, only 12 inches deep as opposed to most 75's that are 18 at least. That way the clams get good light penetration.
For now I would recommend stronger lights before keeping any clams, like steve-s said above. Clams typically feed on photo and zoo plankton particales in the water, and pick up whatever access they can get from the zooxanthellae that lives in thier tissues. Under less than optimum conditions they are stressed from either too little plankton or the effort to create more zoo to compensate for dietary needs.