What is your current Calcium level?
I would stick to weekly water changes if you are using a salt that mixes to an appropriate level, Reef Crystals or Tropic Marin.
If your NSW doesn't mix to 400+ ppm of Calcium, then you should dose with kalk, or a two part solution.
As for lighting, no matter what clam you get, you should place it as high as possible. Unless you can put it in the top half of the tank, and more towards the center; I would not put a clam in there.