Well clams aren't nitrate eaters, they just say healthy with a little nitrate to feed off (5ppm or less). If its FOWLR then you might have some clam eaters or pickers that could kill it. Metal halides is the only way for clams, and this is where they get most of their feeding from. Sure a fuge helps feed the fish, corals, and some inverts, but lighting is the main thing for clams. Reducing nitrates is as easy as water changes, but there are alternatives like deep sand beds and nitrate reactors. Also keeping the fish "hungery" with moderate feedings and good skimming can keep them down.