I actually heard that with small clams, they get more energy from feeding from phytoplankton than from light. Since they are small and their tiny mantles don't provide enough food via xoozanthelae (whatever).
I concur with Zepplock. We have two of those little guys here in a 15g tank under 130W PC lights and they have grown (you can see the growth in the shells). Of course the owner feeds them phytofeast almost daily (he adds ALOT).
I'm sure that in the long run, MH are better, but I think that with phyto feeding, they can thrive even under PC lights. Though I know clam guys are gonna jump me for saying that.
FWIW, I also bought some of those little Maximas that are now living in my tank at home under MH's. One of them went on a death walk, but the two remaining ones are quite nice!
hang in there and get some phytofeast!
V